Make it really obvious what your site does or keep making inconsequential first impressions.
The site is unwelcoming, and doesn't tell me why I am there. The About page is a lot better, but doesn't tell me why I want to use Gravity and not go to pre-existing communities. If I want to talk to people about video games, for example, I could visit NeoGAF or the Penny Arcade forums and talk to them.
If I want to talk about tech, I come to HN :)
Why do I want to go to Gravity? Why do I want to go to it over Quora? (which, BTW, doesn't answer the pre-existing communities question either)
The explore page reveals the functionality, but is overwhelming.
Edit: regarding Gravity: I agree. While I do enjoy video's, because often they get the message across incredibly fast. This video doesn't. It's cute, but once I click explore and see a forum, it's a letdown.
Also, I don't have sound so even if I did give enough of a crap about a site I've never heard of that does some undefined thing to watch a video I still wouldn't have been able to find out.