I think having "Active" as default is unfortunate too, if a new user sees the top post was from two days ago they might assume the whole platform is inactive, since they can't see that a new comment was added just seconds ago from that page view. Having "Hot" as the default would probably be better since those are usually newer posts with at least a few comments.

My biggest issue is that posts stream in regardless of the sorting option you have picked. So, even if I am sorting by 'hot' posts I'll get a stream of posts that where made less than a minute ago.