Sounds trivial, I know. But you are working on the text of your comment. In a small space. So the natural reaction is to get a new line by hitting the return key. Wham, your half completed comment is posted AND you have only 5 minutes to get it right. Up goes the heart rate. Would it not be more friendly to have only the Add Comment button do the posting?
Well...since comments can't transcend multiple lines, there's no real reason to hit the return key until you're done writing it. And if you run out of time, there's nothing wrong with copying the comment you half-started, deleting it, and starting fresh.
It took me a while to figure this out too, but once you train yourself out of writing formatted paragraphs in a comment, then this isn't that much of an issue.
Further, I would also disagree since it's counter-intuitive; there are forms all over the internet that treat the return key as "submit". why would we want to break that in the case of comments? Answers, I can see (and totally agree with; otherwise it would really suck to write answers).