I just happened to be looking at the little icon legend at the bottom of the thread list:
Yellow Open Envelope for new posts.
Grey Sealed Envelope for no new posts.
However there’s also:
Red Open Envelope for new posts (More than 15 replies or 150 views).
Grey Sealed Envelope with rays for no new posts (More than 15 replies or 150 views).
Any reason why we don’t see the second two even on threads that meet the criterion? Just another board feature that went unimplimented to save hampster power?