The problem with a tree is if the discussion gets long eventually several branches reach a point where someone being discussed in one is just a variation of what is being discusses in a different once and you need to bring the two back.

Yeah, there's no good/bad solution to this, albeit I would say that today an hybrid of a vBulletin-like forum with task planning capabilities (ala GH Projects, where pretty much every entity is a GH issue) with a sprinkle of AI you can ask stuff for would meet 99.9% of my needs.

A low-tech compromise that I find preferable to list or tree is what YouTube does, by having one-level deep trees.