Soraka is very strong on Dominion but I think lots of people don't know how to play her. She is the classic "Passive aggressive" champ.
Your job as soraka is to apply constant pressure, wherever you are. Some people only like the flashy champs--like assassins that can blink in and instagib an enemy. Soraka is underestimated partly because lots of people haven't clued into the fact that Silence is one of the strongest CC in the game. Silence prevents enemy champs from executing their combos. Therefore, proper use of silence requires knowledge about all the enemy champs' abilities and good timing. Using it too early or too late is common. Silence on Soraka's infuse is particularly good because it happens almost immediately. Compare this to Kassadin who has to wait for the null sphere to hit the target, or Chogath and Blitz who have to get in close to deliver the AOE silence. It's easier for Soraka to mess up the main enemy killer by applying infuse at just the right, annoying time.
Soraka is a good pusher, but lots of players will push too far with her and get themselves killed. A good Soraka plays off the minimap and knows where all 5 enemies are at all times (or has a good idea). You are tanky with the right build but not so tanky you can survive a gank bottom if you've pushed too far.
Finally, play off the minimap so you can apply Wish at the proper time. If you don't play well, you will tend to use wish to save yourself. It's better spent during a big team fight somewhere, whether you are in that teamfight or not.