I've been playing Dota2 for quite some time now. Not very long on the grand scheme of things, but long enough to know the basic ins and outs of it and how it differs from LoL. I've been playing for just over a month and a half.
And well, despite being told that Dota is so hard and LoL is easy. I find Dota so much easier than LoL. It's just so much more simple to play.
It's in the stats.
In LoL there's a big load of them. AD, AS, Crit, ArmorPen (flat and %), AP, Magic pen(flat and%), Armor, MR, HP, HP regen Mana, Mana regen.
You need to know all about min-maxing your damage. Playing a carry; Would more AD get me more damage, or should I get some phantom dancers? Am I facing heavilly armored dudes and need armor pen? Is more damage even required and I can build tanky? All these you need to make decisions about, if you don't you'll get utterly floored by someone who has.
In Dota there's 3... Strength, Agility and Intelegence. Those are the main stats, with a few sub stats all linked to the main ones (You get HP and HP regen from building Strength, etc). And a VERY few minor stats you'll hardly ever come across except in certain builds like Lifesteal and Magic Resistence.
You can just follow a build to the letter. You'll generally never need to deviate. (Also due to the items being far less stats and far more special effects).
In addition, most items give all three stats anyway (Branches, Circlet, Ultimate orb, Magic wand, Bracer, Null Talesman, etc) so you really don't need to bother choosing except right at the start of the game when you'll probably need some more stats to help with last hitting.
There's hardly any science to playing Dota, it's just keep doing what you are supposed to do and you'll get better.
And then there's how easy it is to get into. LoL has Runes and Masteries. You can be playing LoL for months and months and still not be ready for high level play. You can save up every Influence Point you earn from level 1 to 30 and hardly have a single rune page which makes all the difference.
Whereas Dota has all the characters unlocked from the start and rewards you for playing around with them all. There is no out of game boosting like runes in LoL, it's quick and simple for new players to get a great idea of what Dota is.
(And also DotaCinema on youtube has literally the greatest newbe coverage of any game I've ever seen.)
Dota IS mechanically harder than LoL though. With more item actives means more buttons to push. And some heroes have 6 skills like Morphling. However these simply come down to muscle memory after not too long. (When Ryze's combo is qrqweq, how much harder is 1wq3w?)
Some of the more complex heroes also have more than one controlable unit which you have to miromanage like Meepo and Broodmother. But you don't have to play those until you're more confident in your skills. (And for most of them you can click drag to select all, then right click on the dude you want dead (Broodmother, Nature's Prophet, etc) there are only a very few heroes you have to micro each unit individually (Meepo, Brewmaster))
Invoker's also quite hard to master (although I personally found it much easier than I should have due to playing a game with identical mechanics to him for a long time (Magicka)).
There's a couple more things you need to know about Dota than you do about LoL, such as how to use the Courier and Denying, but again, it's not too much to learn.
That said... I hear some of the things which Dota1 had and yes. Dota1 sounds like one of the hardest things I can imagine. Uncustomisiable keybindings set differently to each hero, (not QWER for your skills, DHAU or something for one hero and GYNE for another), and lack of some things any other game would take for granted.
So yes; I believe Dota is mechanically harder. But has so much less you NEED to know just to win a match.