My preference is interesting threads or ones I can help on. As a result, 90% most of the time, I'm reading through threads just to learn. Probably half of my posts I delete before I hit 'post reply' because I've lost interest in posting by then. Go figure!
Threads that I've already posted in are ones I try to avoid posting in again.
Old threads, I read, but don't post in, since imo, old means people lost interest in continuing them or said all that needed to be said, so I read it and move on. (I really don't care for it when someone bumps an old thread for no apparent reason.) This doesn't mean that many old threads aren't great. They're great! Just done.