Often I see answers like "Thanks!" or "This wouldn't work in IE" that should be comments. Same thing when the OP adds additional info to his question by answering and not editing the original question.
When I see this I usually leave a comment saying "this should be a comment, bla bla bla, kthx", but most people won't remove their answer. I think it's because people making this mistake are new to site and don't check their comments or simply don't care about them.
Since I don't have the powers to delete an answer myself, that's all I can do.
How about being able to flag as "should be a comment"? I know that I can flag for mod attention and leave a comment saying the same thing, but since it happens quite often, why not promote it to its own little radio button?