Soraka doesn't just sit back and dish out a heal every 20 seconds. Get off your @$$ and do something.
There seems to be some misconception about Soraka and how she "Just heals and gives mana" in lane and late in the game. I'm here to tell you how she can be great in lane for wittling opponents down, and in general kicking @$$.
-From here on out, my opinions and view on Soraka, and how she should be played-
Don't get your Q until way way way late or until you need more pushing power midgame... Yeah, I don't care how AOE it is, or how you can spam it. You don't get it for a long while for several reasons. First, Soraka isn't very mobile, this skill roots you to the ground when using it, and is hard to consistantly keep up with someone while spamming. So it's bad offensively and defensively. Secondly, this skill does PITIFUL damage after resistances are factored in. Thirdly, you are going to have to get way too close to keep using it if you plan on doing some meaningful damage. Fourthly, you are going to burn your mana. And lastly, the other skills are WAY better to level up.
Your W is amazing, however ask yourself "Do I need this first?". If you are going up against a kill lane you may actually want to delay it and grab E first for early and constant harass to wittle them down. Usually kill lanes don't get fully functional until a few levels are gained anyway.
Your E is likewise amazing. It's a free-to-use skill that does damage. The range on it is 750 too! That is like, beyond ranged carry range. Level this up when you can, sometimes I max it first, sometimes I don't. It's what is in the game. Plus, leveling this up also restores more mana to an ally when needed (usually give it to them when they are around 40% mana), the silence duration increase helps secure more kills on the offensive, and helps protect allies for a bit longer on the defensive.
For items you HAVE to grab a soulstealer as your third or so item (cause you need the sightstone). As Soraka, you gain pretty much every assist on the map (and sometimes kills by accident). "But, what will she do with the AP?". Heal a $#!+load more for starters. Her two heals have a combined AP ratio of 115% AP, with her 70% AP ult being a multiple person heal too, that is A LOT of additional healing for your team. Not only that, but there is the "Oh, she has a soulstealer? Get her first in fights!", this means less people on your carries, and since support items are generally tanky ones, it can only mean good will come from it (as long as your allies can do their jobs right). The item pays for itself after a few stacks which are easy to get, and I usually average around 8 stacks.
Now I'm not going to say that traditional support items are bad, on the contrary, they work. Don't be afraid to build different items every game. Build GP5 if you think it'll be a long game or won't get many assists/kills. Build the chalice if you see lots of hard cc on the other team. By all means grab a Iron Solari Shield Thingermabob. But if you have some extra cash late, don't rule out Rylai's because it is amazing on Soraka as well. That silenced enemy is now slowed too, seal their fate! It gets a deminished 15% slow from her Q, but you can kinda think of it as a 15% perma slow to the entire enemy team in a teamfight setting, and if you have captian lucidity boots on top of that, it could be a saving combo for someone. The added ap and health help you heal more and survive more as well.
(More to come... Maybe. Bedtime. I was annoyed when a red said most of the time you 'Push Infuse on ally without max mana or you're doing it wrong'. That's not how I play her at all. That's not how I feel she should be played. She can do a bit more than top off bars. She can generate threat and harass pretty great.)