One of my peeves is blogs that appear to allow comments but don't publish them, or only publish the ones they agree with, not explaining their rules. This is especially annoying when I discover this after spending time on a reply.
So far, I allow all comments that are on topic, without regard to whether they agree with me or not.
If someone makes a comment trying to take an internet squabble to the real world--outing a blogger, posting personal information, etc--I'll probably delete that. I will also probably delete posts that are trying to sell stuff, or promote another website more than they are on topic. Comments on fresh posts get published immediately, unless caught by Blogger's spam filter. Comments on older posts get held until I release them.