Imagine that you are a Foo — a member of some arbitrary demographic group. (You can't stop being a Foo.)
Now, imagine that there exists an online community — let's call it Open Minds — that you're moderately interested in. But when you go there, you find that (alongside the interesting parts), viewpoints such as "Foos are not really people", "it's okay to torture Foos for fun", and "non-Foos who speak up in defense of Foos are traitors" are repeatedly aired there by a minority of community members.
Many others in Open Minds disagree strongly with these anti-Foo views; and consider them nasty, false, and uninformative. But for this disapproval, those folks are often denounced as closed-minded — even by others who do not themselves hold anti-Foo views.
Meanwhile, there are other communities, perhaps just as interesting as Open Minds, where treating Foos as non-persons is considered obviously wrong both as a matter of moral norms and as a matter of self-evident fact. In those communities, a person who expresses the idea "Foos are not really people" thereby excludes him- or herself from reasonable discussion. That person is considered a troll or an asshole, and possibly banned if they don't shape up — or, at least, shut up on that particular topic.
Given that you are a Foo, where would you choose to spend your time? In Open Minds, the community where a small minority repeatedly calls you a non-person, and "open-mindedness" is taken to include considering that possibility? Or in the community where calling you a non-person is considered to be obviously wrong?
(Consider also that you know that you are a person, and that it is not okay with you if someone tortures you for fun. In other words, you know that the anti-Foo views are false. As far as you are concerned, those views aren't a matter of abstract speculation; they really are people using obviously false ideas to justify doing horrible things to you and others like you. Besides, you've heard those ideas before, and you don't learn anything from hearing them again.)
Now, consider further that Foos may have particular experience or information that non-Foos lack — merely because two paths through the same territory do not yield the same map. (It isn't that Foos are better or smarter than non-Foos, and it certainly isn't that everything Foos believe is true ... just that they have had access to different data.)
In effect, Open Minds has chosen to prioritize ensuring the community's access to anti-Foo views over ensuring the community's access to any information that Foos themselves may possess.
How open-minded is that?
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.