In my view, a good idea is generally the merging of one or more crappy ideas and so-so ideas into something that's workable. Because of this, I think you need to provide some mechanism for the cream to rise to the top. For the use case I'm envisioning, sharing ideas with a small number of users, I don't think a traditional voting system would work.

I think this could be an interesting space to employ some sort of visualization. I'm thinking something like http://www.archimuse.com/mw2006/papers/lowndes/lowndes-fig2-... for ideas.

Technically, I'd want something like this to be real time, shouldn't be too hard with ajax. There would also probably need to be supporting collaboration features, like chat.