I rarely open the comments here. My blog is a small one -- a very small one. I don't even bother to promote it. Basically, it's my own space to hash out ideas. I don't have a "community," nor do I think of myself as some kind of thought leader, so whether to open the comments or not is a non-issue. On the rare times that I do, my policy has always been "don't be an asshole." Interpret that as you wish, but I think it's pretty self-explanatory. Mostly, I just don't want to make this blog a job.
Lately a lot of the blogs I read have been taking stock of their comment policies. Blogs with an activist bent have pretty definite rules about what should or shouldn't be said, which ideas are acceptable, and who "gets" to talk. Within certain ideological parameters, those things are fine, even expected. However, the world isn't a gender or ethnic studies class, most people have never heard or critical theory, and no one I know outside the social media world looks at privilege as anything other than economic privilege. These are ideas to be challenged, of course, but not bullied into submission by their supporters. People need to make mistakes.