In general, I an a person who likes to debate, discuss, persuade, convince and, in the nicest possible, literal, sense exchange words. However, I find that forum debates on the internet tend to frustrate me. If I join the debate on a site's forum, I almost always end up spending less time on that site in the long run (there are exceptions, but as a general rule that's the case).
I think there are two reasons for this. One is the often cited fact that we don't tend to see people as people when we interact over the internet. It's not difficult to begin a discussion with somebody and then leave it hanging when 'real life' gets in the way (I've been guilty of this one) or just because you're bored. That's not really conducive to a meaningful discussion.
The other thing that I think frustrates me is that internet conversations are written down and recorded. That makes it incredibly obvious if your argument is inconsistent. In a spoken conversation, I can believe that you've just forgotten what you said about x five minutes ago. If it's written five centimetres up the page, you should reread it even if you don't remember it and I think it ought to be easier to remember it. People still contradict themselves though.
Because of that, I don't tend to do the forum thing. Comments are another matter though. Having a focus, a blog post, a picture or what-have-you, seems to keep a discussion more careful. Even an off-topic comment thread often has more rigour than a forum thread. I suppose the quality of the original item commented on might dictate the quality of the discussion too.
I do enjoy discussion, but I think I'll keep it to the comments, to avoid frustrating myself. Because while I enjoy discusions and debates, arguments and fights are a different story.