I think the world rightfully has accepted the linear thread in discussion forums. The negative of this is that people quote entire, untrimmed, posts and that's annoying. (Of course, lazy asses did this on Usenet too, but whatever.)
Is there any lingering preference still for the Usenet-style tree of discussion? I personally always thought it sucked because it's not how human beings talk. It's like speaking to a room full of people and eventually having to work the entire room, one person at a time, and simultaneously conduct a zillion different discussions.
Digg does an almost hybrid approach to this, in that they only allow the branch to go one level in.
What do you think?