I don't think the novelty explains very much, the digg/reddit comment tree format is a clear improvement in the sense that it makes it easier to find and track interesting discussions. I always liked the aspect that you could follow a coherent back and forth where the people carrying the conversation tend to change with each comment. Even with all its problems, I can't think of another format that can match it in terms of sharing the spotlight among a diverse set of voices.
I could never really get into the twitter format because it seems to be about a particularly spicy take followed by long string of replies to that take, at least without additional clicks that completely change the context. Its single virtue seemed to be its departure from anonymity which allowed it to be a showcase for voices that were already influential within society.
The oldschool forum format requires a lot more scrolling and superfluous content that is unrelated to the discussion, and it is hard to go back to once the wave of nostalgia passes.