In posts where there are a lot of subthreads or even just two or three subthreads that have a lot of responses to them, the ability to collapse them down would be nice. Collapsibility (word invention!) would be nice for all subthreads that have a response to them, but even just a plus/minus symbol on the main subthreads would be great.

Early days, I know, but I just wanted to put that out there for whenever you get around to it.