It's kind of a subtle thing, but I think the hierarchy of topics (listed newest first) and posts (shown by topic, oldest-first) matters a lot. Creating a new topic is a much more thoughtful and intentional activity under that model than when your "room" is just a continuous chatbox with replies often accidentally posted to the main instead of being properly threaded.
Categorisation tends to be better.
And each post tends to compete in a page view of all posts, versus a stream of things in many cases.