I think that if I didn't know anything about LessWrong, the first version would be the one that would be more likely to attract my attention. There are a lot of websites that make vague promises that reading them will improve your life. The first version at least gives examples of what exactly this website is about and how exactly it is supposed to be helpful. While the second sentence repels no one, it seems unlikely that it could pique anyone's curiosity. That might not matter when you personally recommend LW to someone, since your recommendation would presumably be enough to make them think that there is some content behind the vague introduction. It seems to me that we might even have an interesting situation when vague introduction might work better on people who were recommended to check out LessWrong by other people (since it wouldn't repel them), but I hypothesize that concrete introduction would be better for those who encountered LW accidentally, because while most people wouldn't read past introduction either way, it is the first version that seems to be the one that is likely to catch at least someone's attention.
By the way, I am not sure if having what is basically About page as a frontpage is the best way to introduce new readers to LW. Few blogs do that, most put the most recent posts on the frontpage instead. That makes them look less static.
I told an intelligent, well-educated friend about Less Wrong, so she googled, and got "Less Wrong is an online community for people who want to apply the discovery of biases like the conjunction fallacy, the affect heuristic, and scope insensitivity in order to fix their own thinking." and gave up immediately because she'd never heard of the biases.
While hers might not be the best possible attitude, I can't see that we win anything by driving people away with obscure language.
Possible improved introduction: "Less Wrong is a community for people who would like to think more clearly in order to improve their own and other people's lives, and to make major disasters less likely."