It is sometimes conversation, and that's great. It is much more often argument. And it is frequently pointless argument between two people who don't want to learn anything but instead are playing "king of the hill" with words.

For people who really want a conversation, I think a forum should support some way to privately contact one another. And a good ability to start new discussions. But for general-audience, broad-appeal forums, I think threads usually create more problems then they solve.