I dunno, I have seen a few threads where I was somewhat stunned at how stupid the OP was. Like this one about racism you locked. I would think what is meant by stupidity is people who don't research their claims or who make really bad arguments or who ask questions Google would be better at answering. But I don't get into forum conversations with people who I think aren't (shit I sound like a dick now) clever enough for me.
Not that I think I'm some amazingly intelligent person or anything, it's just only a few people disagree with me in such a way as to pose arguments I've got to actually think about. Like, in some arguments it feels more like I'm explaining very basic ideas and concepts that seem stunningly obvious to me. The one that comes to mind is the old 'why aren't atheists immoral' topic but there are probably better examples elsewhere.