[EGreg]

Why not simply have threads be channels that are children of other channels?

This way you discourage branching but allow an unlimited amount of it. And each thread has the same properties and rules as a channel.

All you have to do then is solve the problem of listing the channels in the sidebar, which the user is participating in, in a hierarchical way. A user should be able to leave a thread and it would disappear from their sidebar. Simple.

What's wrong with this model?