The stronger forms of evolutionary psychology and human-diversity stuff. High confidence that most/all demographic disparities are down to genes. The belief that LessWrong being dominated by white male technophiles is more indicative of the superior rationality of white male technophiles than any shortcomings of the LW community or society-at-large.
Any and all neoreactionary stuff.
High-confidence predictions about the medium-to-far-future (especially ones that suggest sending money)
Throwing the term "eugenics" around cavalierly and assuming that everyone knows you're talking about benevolent genetic engineering and not forcibly-sterilizing-people-who-don't-look-like-me.
I don't mind #3, in fact the discussions of futurism are a big draw of LessWrong for me (though I suppose there are general reasons for being cautious about your confidence about the future). But I would be very happy to see #1, #2, and #4 go away.
I find stuff like “if you don't sign up your kids for cryonics then you are a lousy parent” more problematic than a sizeable fraction of what reactionaries say.
A long blog post explains why the author, a feminist, is not comfortable with the rationalist community despite thinking it is "super cool and interesting". It's directed specifically at Yvain, but it's probably general enough to be of some interest here.
http://apophemi.wordpress.com/2014/01/04/why-im-not-on-the-rationalist-masterlist/
I'm not sure if I can summarize this fairly but the main thrust seems to be that we are overly willing to entertain offensive/taboo/hurtful ideas and this drives off many types of people. Here's a quote:
The author perceives a link between LW type open discourse and danger to minority groups. I'm not sure whether that's true or not. Take race. Many LWers are willing to entertain ideas about the existence and possible importance of average group differences in psychological traits. So, maybe LWers are racists. But they're racists who continually obsess over optimizing their philanthropic contributions to African charities. So, maybe not racists in a dangerous way?
An overly rosy view, perhaps, and I don't want to deny the reality of the blogger's experience. Clearly, the person is intelligent and attracted to some aspects of LW discourse while turned off by other aspects.