I agree with Denise about sorting threads by date of last reply rather than date of first post. If a topic's already been discussed at lenght, why make someone summarize all the previous replies just to give it exposure? I've replied to old topics [e.g., People/Singles/Insurance] that were so deeply buried, no one ever read far enough down to respond. Any topic that gets a new reply should float to the top, IMO.
Sorting by first post is useful for short-lived threads [e.g. all the Good Morning! discussions]. The exchanges last a day or so, then the topic is properly buried. But longer lived discussions need to be revisited occasionally if someone has new information to contribute or never received a response and wants to bring it to others' attention again. Sorting by last post works best there.
I guess it's a matter of preference. If someone wants to keep banter discussions on top, sort by first post works best. Others whoe are interested in most recent discussions on all topics prefer to sort by last post. It would be nice if we were able to customize our preference as with the flat view/threaded view option.