What do you mean by "thread"? Isn't that the same as a "topic"?
It's basically like your average OS file system. If you were looking at a folder on your computer, you'd have the main folder (the initial post), with each reply to it being its own folder. Replies to replies nest down another level.
It... honestly, it's not much better for readability or usability than what we have here, t'me. It's different, and it has some advantages in keeping track with one line of discussion, but when you've got several or a lengthy back and forth it frankly becomes ruddy annoying to navigate, and multiple discussions branching off the same topic doesn't so much lead itself to coherent discussion continuing as discussion dying quickly as no one is quite sure what everyone else is saying. It's maybe better for something with
major traffic (though it leads to just as much being lost as something like simple machines), and it's alright for
really small discussions with only a few branches, but basically everything else, well.
It's a lot like trying to navigate a particularly convoluted game directory looking for a specific file or folder, with every bit of awkwardness that entails.
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Not sure what a better system would exactly entail, really. I do like what Xenforo is doing with threadmarks, at least. Took a while to get used to, but it can make thread navigation significantly easier (providing the OP is staying on top of things, anyway). What an improvement would probably boil down to would be multi-format integration (allowing for the OP or maybe particular subforums to use different formats), or maybe just a preference option to change the format for the individual user.thread