Some business ideas are innovative, in the sense that they are a new, bold, or risky approach to solving problems. I don't think you have anything to lose by sharing those ideas. These ideas are all about execution, and passion and domain knowledge will out. Share these with impunity, I think.

Some business ideas are just really fancy arbitrage. Execution matters, but the value is self-evident; the more people that know about the opportunity, the faster the market is going to get flooded. I'd keep these a little closer to the vest.

I suspect the start-ups that we talk about and focus upon here on hacker news fall more in the first category and less in the second.