Okay, Twitter is more like a disassociated bag of nodes in a partially connected non-directional graph where half the edges don't connect to the node they're intended to connect to.

YouTube is a pack of raving howler monkeys with the occasional howler monkey researcher at the periphery. And, in fact, YouTube used to have threads, but they were still threads full of howler monkeys.

I'm talking more like your classic bulletin boards producing high quality content, like MetaFilter, Something Awful, and the like.

We agree on the sites, but I don't see what the conversation being flat has to do with it. Reddit is famous for the AMA threads, those are threaded.

Yeah, but try reading an AMA after it's been finished. It is really hard to find everything from the Askee.

I'm not intrinsically opposed to threads, but I'm not sure they're super beneficial outside of technical contexts or in an environment where people are expected to break a larger conversation off into it's own little set of discussions to keep the high level conversation easy to understand.