1. Pretty good presentation. Looks clean, professional.
2. Easy to understand what your site does and the value proposition. This is a huge step in front of most major sites.
----- Here is my criticism:
1. Your demo page is too tailored towards first time users. You need to look at models like reddit and digg to come up with a nice balance.
2. Like everyone else is saying, don't require sign-ups. And if you must, use OpenID and FB Connect to streamline the process.
3. Pretty pictures. Use some nice flash charts to give a better understanding on how your predictions are trending.
4. To be brutally honest. i don't see this site making critical mass. The problem with web 2.0 is that most people don't have time to join so many social networks, you need to consolidate this as either (a) an API for other companies to use and make your business a b2b model (there are a bunch for predication markets already) or (b) make it a social network application (but there should be a couple of these already too)
http://www.cnn.com/ELECTION/2008/veepstakes/index.html
That's a good example of how to leverage some cool algorithms and a funky API to integrate with major sites.
Perhaps I'm a dick wad, but I think honesty is better than sugar coating. If you want to talk further, e-mail me charlesju[at]gmail[dot]com.