I'm a conservative, so I might be biased, but the notion that Lesswrong is culturally unwelcome to lefties strikes me as not just wrong, but funny. In any given scan of the site, I'll see 3-4 things that offend me.

Threads will contain, not as the point of the thread, but just as background noise, as assumptions with which the writer presumes everyone will agree, atheism, pro choice stuff, polyamory (usually with same sex stuff relationships in there), discussions of how cool it will be once we turn out bodies into robots, etc.

I recognize that it is possible that the site is somehow also offensive to progressives and I simply miss out on all of the conservative talking points because they are transparent to me (fish don't see the water that they swim in, etc.), but I don't think that's the case.