I'm only a single data point, but you've lost me because I have no idea what gravity is upon first glance at the site and I don't want to watch a stupid animation of stick figures talking gibberish.
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.
I didn't even get that far. I don't like watching videos on the Internet, mainly because I usually have other music playing, and they don't play so nice with linux. I don't want to have to stop everything I'm doing just to see what some - probably irrelevant - website is doing.
Get earcandy. It automatically fades music players once a flash video or a video player starts playing: https://launchpad.net/earcandy
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.
The thing is, I don't want to have to stop my music just because someone wants me to watch their video. I like my music! What's the big problem with text these days?
Another one here! Not even a quick slogan to give some idea why I might want any type of gravity apart from the type that keeps me stuck on the world's surface.
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.
As others have said, the video tells me nothing so I look at the about page - the description sounds great but provides no incentive to sign-up. You need to get me interested in a topic straight off the home page, without having to register.
Close, but I'd agree with ihatemummies in that it's closer to Yahoo Answers (at first glance at least). It seems more like a collection of forums (although I'm stretching that description as well).
Apart from hating the video, I also didn't sync my Twitter "friends" because gravity asked for write access. Reading should be enough to find my friends.
Make it really obvious what your site does or keep making inconsequential first impressions.