What's the incentive to share ideas? The satisfaction of getting them off your chest to people who build businesses off them? I doubt very many people would do that. I would only ever consider submitting my ideas to two types of people:

1) very close friends/potential cofounders

2) people who can help me build it (investors, etc..)

I don't think this site has the right interface because people are naturally protective of ideas that can make money.