New topics should be split into their own threads, and when people start using the forums right, that will be easily done (there is an option to split a post and all it's child posts)
So when a conversation goes off into its own little branch of sorts, you would just "break it off" into its own thread? That'd work pretty well.
As it is, I like how this is going - the only thing I don't like is the "depth" of replies (hybrid mode), I wish that there was an option where you can set that higher, much like you can set the number of replies per page..