Hey guys!
Recently, a summoner spell named 
Flash became hot topic (at least at this wiki, but I've heard some words about it also out of wiki, though I visit such sites as Reddit etc. rarely). The reason of this is mostly same - "Flash is bad and imbalanced". So, here I come with my own opinion about all that is said about the spell, balance that is chained with it and some other things that are commonly considered as imbalanced/OP/etc.
Some people just don't get the arguments and recklessly go against maths and logic "omg I can't handle it so it's wrong", some players put the fault on lack of skills "you just suck, that's all", others don't even care "just keep playing lol", few people bring their own balance ideas that can be discussed as great or not "let's rework it". This is what goes now around Flash.
I remember exactly same situation that sticked to something else. Maybe you too remember? Remember what was going with 
Noxian Guillotine? Yes, it's perfectly! same situation.
When 
Darius was released, community instantly went insane with him being too OvahPowarah'd, exactly focused at his ultimate. And it was going on and on. Then I realized that something is wrong. Something that I couldn't see without taking a deeper look.
So what was wrong then? At first, people just begun to lose their minds and kept failing against him. Then, some people came in with a new state. State "you noobs, you just don't know how to counter". Never-ending ideas about counters stepped in, let me introduce "teh counturz dat do epic winz": 
Fiora [hahahaha], 
Teemo [that dies instantly after hitting 6th level and his range actually isn't safe], 
Garen [which is, in fact, countered by Darius], 
Kayle [this one is good counter imo, but requires skill and good reflexes]. Four most popular ones. For some time, you could hear "it's perfectly fine, he is easy to counter" even if, in fact, people did not countered him at all. The only reason why it was so OK is psychological effect. You tell people that they can handle it in this way, so they think they really can, and if they actually did well before, they'll think that it's your method that helps them, and if they still fail, they'll follow your method and think "it's perfectly fine, I have to try again". It shouldn't work at all, but any "intelligent" idea was spread very quickly and if one hundred people tell you it works and they are not going to think for yourself, how are you going to tell them they're wrong? You can accept it or say that you've accepted it and keep your opinion for yourself. At least, that's how almost every player did. If you said that he is really strong and nothing changed at all, people just flamed you down to hell in few seconds.
Some time passed. And what? Surprisingly, no one said that it's too strong. Everybody used "toxic" word instead. It wasn't good, it was toxic for gameplay because you could recklessly steal all stuff around. And Riot says same thing.
Everything that people can't handle - doesn't matters if it's really OP due to its mechanics and usage, not numbers or just requires skill to fight against - becomes "toxic". Noxian Guillotine. Proxy Singed (I can write something too, but the post is about Flash, not Singed). Now, Flash.
"Toxic" is the key word.
Let me explain what is the thing that I've considered as "wrong, but I couldn't see it without deeper look".
The bad, hidden thing: There are two types of "toxic stuff" I've seen. First one, something that requires from player not being a brainless bastard < Noxian Guillotine> and second - stuff that requires low skill yet can give high reward <Proxy Singed, Flash>. Both of them could be absolutely OK, but over all the rage, people are not even trying to REALLY do something to improve performance with/against. THIS is wrong. Everything is balanced as long as it doesn't requires deeper tactics (that are still easy to find out). Take a look at 
Jayce. The unbeatable top laner. 
Elise. The % of health stuff monster of top lane. Cpt Tito. I hate him at top cuz he pokes with AA too much. Kayle. It's hard to perform well against her. Four champions who have high win ratios on high skill levels (maybe Jayce fell down now). What is the thing that combines them? RANGE. Built-in range of bursting or poking. When top meta is supposed to bring in low range champions that have to stick to the enemy. All of them considered as problematic (except Jayce that got nerfed in wrong way [SURPRISE ANOTHER BAD NERF]). How do I look at them? Just another top laner, I just have to play in different way. I'm still shocked that almost no one, even in high level gaming got the idea how to play against them, while I found out simple thing to counter all overranged enemies, and actually understood that you should ALWAYS do countering, and you can do countering even if your opponent is considered as your counter pick.
I quite often *cough*almostallthetime*cough* see that people do not change anything in their playstyle, go against something that is considered as OP and just lose if player that wields "teh OPness" isn't full retard, which is rather obviously not going to happen in high tier gaming. For example, those four champions. Sometimes people try to counter pick. That's all.
Dear people, if you try to counter pick, then you also have to counterplay and counter build or else you're doing nothing more than randomly picking a champion. Also, summoner spell countering is something you can't forget about. You are not supposed to take Flash + Ignite every time.
If I'm facing Elise/Teemo/Kayle/Jayce, I do counter building by taking 
Doran's Shield and potion as first items while against Teemo/Kayle/Jayce or two 
Rejuvenation Beads + pots (not only health pots) and a ward if needed against Elise. Instead of 
teh tonz of troo demuj, I'll take 
Exhaust which perfectly closes the gap if I decide to attack. Now, I'll get poked hard, but hey - I've got massive regen, so poking effectiveness is reduced. Teemo, Jayce and Kayle can stick their basic attacks in... somewhere safe, teh shield reduces the damage taken pretty much. If I decide to attack, I'm not getting kited because Exhaust closes the gap. The question is, does it works? The answer is, try it and see if it works.
There is a huge difference between overall countering and just counter picking.
Coming back to the key word "toxic", how Riot fixes something that is considered as "toxic"? Simply - nerfs something that does not matters or nerfs the thing in wrong way, wrecking it down to trash tier.
Nerfing the thing that does not matters, example: 
Noxian Guillotine. It's the same true damage as before. The only thing that changed is the fact that it's not ready all the time. Now it's 120 seconds in 6-10 levels phrase. Two minutes. It makes almost no difference.
To hit it even harder: Riot wanted to nerf this ultimate because of "too toxic [key word] plays caused by it (simply stealing kills)", so they made it in way that ulting in teamfight puts it on 12 second usability, forcing the player to kill something, or else it'll go on cooldown, simply making player dependent on stealing...
Want to get stunned to the ground? Late game cooldown went from 100 seconds to 80 seconds. Able to begin teamfights even more often than before.
And you know what? Riot said it's fixed, so people think it's fixed and Darius went to trash tier. Even if his power wasn't depending on ultimate only and simply comparable to strongest offensive tanks.
Nerfing in wrong way, pushing something to trash tier, example - 
Olaf. Are you going to tell me that 20% of win ratio drop (highest ratio drop ever if I'm not wrong) is right? They are now going to re-buff him now, and if I heard right, at least this time it's good buff. He was also considered as toxic because of "no counterplay and low skill, high reward".
While I absolutely understand that something may be strong and harder than average to play against, I just don't get how is it possible that "toxic" stuff gets nerfed in wrong way, still being "toxic" in focused place, but people do not fear it anymore. It's like somebody faces 5v1 fight and one of 5 players goes back, and then the alone guy says "it's fine now!".
The last thing I want to say about, our beloved "free out-of-jail-card", the *dam dam dam* FLASH.
I've seen some stuff said about Flash, and that's what I want to talk about - explain why Flash isn't OP.
Some Stuff #1: "Flash is free alternative for saving yourself from a bad play"
It's lifesaver as much as 
Barrier is, but just works in other mechanic. Simply, Flash causes you to extend gap between you and opponent/s. If they don't have anything to close it, you're alive. Barrier extends damage gate enemy/ies have to reach in order to reach your death. If they can't do it, you're alive. Those spells do the same thing, but work in the different mechanic. Both of them can be used in offensive action. Yes, Barrier can be used to safely engage the opponent, making offensive use. In true reality, it's more common to use mobility booster or CC to close the gap than pull more damage than your combo can do. Simply, Flash is less safe than Barrier, even if you won't admit it.
Some Stuff #2: "Flash rewards brainless playing more than tactical playing"
This is impossible. If something rewards bad use, then it has to reward good use in same way or better. Never worse. If it goes about overall skillcap, then yes, I admit too that Flash doesn't has high skill cap, but further, for example, how different skillcap Ignite or 
Ghost have? If Flash is "aim with mouse and press button", so how different is "aim with mouse and press button and be happy from free damage and healing effects reduction" or "press button and get increased movement"? Flash, though you may not believe, has one of highest skillcaps if it goes for summoner spells. The reason why is it easy to use is fact that all summoner spells are just simply easy to use, but some of them will punish bad play more than others, namely badly used Flash or 
Teleport can throw you into hands of enemies, while Ignite is only wasted without any further lose (and it's always less damage somebody else has to do if decides to attack!) and Ghost can be also used for instant retreat.
Some Stuff #3: "Flash makes jungler's work much harder, making ganks pointless because it's available for everyone"
Indeed it does like every other mobility booster or gapcloser! It's available for everyone for some reason too, because if it wouldn't be, you would have to stick to your tower for whole laning if not having a built-in gapcloser, making such champions much worse. And don't tell me they would pick Ghost instead. Simply, danger of getting CC'd and bursted requires instant action. Ghost is much more effective for longway chasing or retreating a lane fight. Just gank again before Flash is ready again.
Some Stuff #4: "Flash combined with gapclosers is insanely strong"
So, no one actually notices that it's more likely champion ability's fault that can get around 750 range within something like 14 seconds than 400 range 5 minutes of cooldown Flash? Even more, Flash actually makes fighting against some gapclosers possible.
Some Stuff #5a: "The only way to counter free blink is to have your own free blink (directly about Flash)"
So how fair then is having free damage output from Ignite? Is it possible to counter only by having Ignite by yourself? Is free strong CC from Exhaust counterable only by another Exhaust? That's why spells are different and should be taken in different ways - it's not my fault that community sticks to Flash like they can't live without it.
Some Stuff #5b: "That's the problem - if people would think with different summoner spell picks, it would be less problematic!"
I quite agree to this. If spells were chosen different without clearly higher pick spikes, then it'd be better. It goes for all spells. Ignite from every top/mid laner seems quite annoying too, and Exhaust meta would probably gives us all nerves too. Balance and proportions, that's it.
Some Stuff #6: "Flash has too big range/low cooldown, making it too strong comparing to other spells"
400 range is returned by Ghost after ~3 seconds. Exhaust mostly closes the gap before its duration ends. 5 minutes of cooldown is even a bit too much imo, but as much as this spell is hated already, I think 5 minutes is best choice.
Some Stuff #7: "Riot builds balance around Flash"
In some point, Riot has to think about Flash while balancing stuff. But giving a champion that has no gapclosers, for example, high damage does not makes him balanced around Flash, but around THE GAME. High damage without increased mobility means you are not all-eating machine and other players can retreat a fight against you. Imagine what would happen if you could get an enemy all the time and out-damage him every time. Is Flash causing the mentioned situation true? No, because it simply has 5 minutes of cooldown and quite low range, so you have to use it with some reason, and actually gives no gapcloser damage dealers chance to kill something at higher tier games and causes them to not be closed in farming-turret sitting circle if camped by enemy jungler.
Some Stuff #8: "Move it to item effect or anything else than summoner spell"
As much as this may work, there is currently no perfect idea for this. Mineko Charat Lucky recently proposed idea of moving it into different boots, which was one of better ideas about Flash reworks, but it also had some issues that would make game probably even less balanced, even though I appreciate this idea.
Mineko's rework concept here
Some Stuff #9: "Flashing over wall causes Flash to be OP"
This is exactly the only thing that we can consider as OP if we are talking about Flash. For any champion that can pass over walls with built-in gapcloser, it does not makes any difference from average flash usage, but if someone can't cross the wall, then it makes quite a mess. Praise Morello that it can't pass huge terrain elements. Also small note, if talking about terrain flashing: Removing Flash would make few terrain-creating abilities such as 
Jarvan IV's 
Cataclysm as absolute killer against some champions.
Some Stuff #10: "It's impossible to win a game without Flash in team, and laning without it is very crippled"
Excuse me, but have you ever tried it? By picking different spells you gain different results. By having 5 Ghosts, you'll always engage the teamfight you want, as well disengage too. By having 5 Flashes you'll be more safe in laning. Simplicity. Winning a game without Flash in team is possible, and is even more likely to be at some point since people often don't have any idea how to play against it.
That's enough writing (it's quite huge post!). Either you agree to my words or not, feel free to discuss it in comments.