Tree-like Forum - Best way a forum can be with repetitive conversations sprung from one question but emerges to look at side conversations. Its organic, allows conversation in a humanely form (since you discuss lots of things simultaneously).But sometimes takes so much of side conversations, that filters your main context, and can be tough to track,people miss the main subject, more noisy,and may strive to tie to the main goal.
Metaphor: Its like in a kids school. Tough to get answers, but striving to!
Flat Forums - Its a Q&A that deliberately gives one's viewpoints. Say, I want you to tell me if I can invest in Stock A,B,C
Metaphor: This is more of viva-voce or interview talk. You have one chance to say that and please move on (We are time constrained or result driven)
Stackxchange - This is just alike Tree Forum, here the difference is that any noise that emerges or cross-questions are stacked in as comments. So you best know these are side conversations. It is more clearly restrained and best organized. Best of both the two earlier worlds, in silos.
Metaphor: This is alike patient treatment/diagnose. Doctors speak on the main subject to diagnose/but will allow side-talks of situations, questions about treatments and remedies or parallel cases to get to the final solution - may be best!