I've been playing a few games with Soraka lately and one of my biggest complaints about her is that she's just not fun to play. I'm used to playing more support roles in games like these and one of the things that makes it a lot more fun is to mix a decent offense in with your support. Take omniknight for example: he can sit back and heal someone low, or if you wait for the right time you can contribute pretty significantly to the damage of the team while still providing amazing support.
Soraka provides such significant buffs to the team that she can very easily be a game-changer. I think a bit of a redesign would go a long way to making her still provide a large amount of support while still being interesting and fun to play. In her current incarnation you're basically just spamming cooldowns as soon as their up on whichever target needs it. The lack of timing needed to excell at Soraka makes playing her get boring quickly.
Also I'm not sure what people are talking about when they say she's succeptable to 5man ganks. First of all, almost every champion is going to get rolled by a 4-5 person gank. Second of all, what Sorakas sit around in a lane by themselves? Teams that know what they're doing with Soraka are going to stick together, push early, push hard, and never stop because they have infinite mp. If you're farming in a lane by yourself to get ganked, you're doing it wrong.
Here's some feedback I have on her current skills:
Her passive is really boring. A lot of characters have interesting passives that really set them apart or add to their playstyle. She gives nearby champions a static 16 magic resistance throughout the entire game. Sure, it fits her playstyle a bit, but it doesn't really do anything an item wouldn't do, and the lack of scaling is pretty different from all other passives.
Her starcall ability plain sucks. If you get it early to farm, you're sacrificing huge amounts of support with early levels of mp refresh and regen. Also, she is not item dependant at all, so she doesn't really need the farming ability. Even at max rank, it does a pathetic amount of damage. In a teamfight, why would Soraka want to be close enough to hit this spell multiple times? She'd be more vulnerable to CC and burst. She's far more valuable to the team staying back to provide constant regen and mp restore and a clutch global heal at the right time.
Her regen I think is fine. It is strong early and mid game and suffers late game. Perhaps to combat this the armor it provides should scale more? Giving 30 armor at all ranks seems rather odd considering how most spells scale.
Her mp restore is overpowered. Soraka will always lane with someone with a spamable move, and that character will have infinite mp. It completely dominiates whatever lane she is in. It makes pushes possible that would in no other way be possible. The team with Soraka will never have to return to the fountain to get mp, and with infinite regens they likely won't have to go back to heal either. The weakened players can just stay back for a few regens and then rejoin the fray, still offering support depending on their skills.
Her global heal is strong, but I don't think it needs to be changed at all. The 2:1 AP scaling is very nice given her poor ability to farm.