"Eugenics" is a problematic word because it's now associated with involuntary sterilization and Nazis.
Is the association unwarranted? Even when eugenics was very much "a thing" in many Western countries (including, AIUI, the Progressive-era U.S) it always denoted coercive intervention to suppress fertility in the unpopular outgroups du jour.
And apparently, Yvain's use of this term is what ticked off this person in the first place. We should just stop using it.
No, not unwarranted at all. It's just that the original definition of the term didn't include coercion, and some people insist on sticking to this "official" definition, history be damned.
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.