RationalWiki (or more precisely its admin) has a strong opinion on which political opinions are the valid ones, and treats other political opinions as pseudoscience. Debating your political opponents is considered a bad thing, because you should not provide them platform to explain their beliefs. The proper way to discuss people who disagree with you is using the "snarky point of view" (yes, this is the official policy).
Scott Alexander is the opposite extreme. He tries to find the good parts even in the Time Cube (yes, literally). Although Scott himself mostly has the usual liberal opinions, he does not enforce them in the comments; he usually only enforces civil behavior. The result is a highly intelligent forum where liberals talk to conservatives, Christians talk to atheists, etc., mostly in a friendly way, on a wide range of topics, some of them political, some not.
Each of these styles is attractive to a different type of personality. Sadly, the former often have a problem with the latter.
I honestly don't know. RationalWiki seems to hate on anybody even vaguely associated with LessWrong, up to and including misrepresenting their ideas (or outright lying about what people have said and done) so they can more easily criticise them. Their articles are worth reading once, to get a list of Bad Things™ about a person or idea, which you can then filter down into actually true stuff at your leisure.
It's frustrating, because some of the criticism is actually warranted, but they just fill such articles with so much banal nonsense, like "Scott Alexander criticised communism, but also studied something the USSR did… :thinking_face:" (paraphrased) that the signal-to-noise ratio approaches Uncyclopedia's.
It's really frustrating, because the articles about topics they haven't got a bee in their bonnet about can actually be insightful introductions, beating out the Wikipedia articles. Nowadays, though, those are few and far between.
https://rationalwiki.org/wiki/Scott_Alexander