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."
Preventing existential risk is part of what this site is about. Do you think it shouldn't be mentioned at all, or do you think it should be described some other way?
I think mentioning it early on sends a bad signal. Most groups that talk about the end of the world don't have very much credibility among skeptical people, so if telling people we talk about the end of the world is one of the first pieces of evidence we give them about us, they're liable to update on that evidence and decide their time is better spent reading other sites. I'd be OK with an offhand link to "existential risks" somewhere in the second half of the homepage text, but putting it in the second sentence is a mistake, in my view.