Not a bad start and I quite like the concept.
Here's my two cents.
Color - check out this site for some example color schemes.
Design - it's a bit blocky; consider adding slight border-radius values to the buttons using CSS. Think of putting a page width and setting margins to auto to center elements.
I would definitely add some screenshots of what a logged in user sees on the main page. Maybe some statements saying what the site is about. Why would a user register if they have no idea what they're getting?
Again, a great start, and with a few simple changes it could help attract users.