I'd like to think that VC's and the judges at YC have quite a lot of experience knowing who to pick, what characteristics to focus on, among other things. I also think money is one of the least important factors into joining YC, as what's more important is the mentoring, along with the network you become a part of. Finally, PG and the other mentors have put their name behind YC, but when you start to allow in companies that win popularity contests, you diminish the brand.
I do like the idea, and it's why I've helped people build websites that can do this (although I need to wait for the JOBS act to finish its evaluation by the SEC), and I've also signed up for Wefunder, a site dedicated for stuff like this. I just don't think this is a good fit for an incubator with the prestige of YCombinator.