Alas, such a philosophy probably repels some potential participants who would otherwise make intelligent comments.
But the 'safe space' policy also repels potential participants - so, it's basically a wash. And only one of these policies is epistemically problematic - I'll let you guess which one.
Giving honest/blunt feedback during class discussions or on papers can cause a very negative emotional reaction in some students.
Obviously, when giving public feedback from a position of authority (being the course lecturer), you need to be quite thoughtful about the connotation of any statements on your part, specifically your impact on the students' perceived status. It's less clear that this would be a problem at LW, where few people speak with any overt authority and the karma system is an independent source of merit/status.
But the 'safe space' policy also repels potential participants - so, it's basically a wash.
When you repel one member of an over-represented group and attract a member of a previously-absent group, you keep the same number of participants but increase the amount of information present in the discussion.
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.