The main thing that makes Unsolved Mysteries different
In my opinion, it's the soberness and lack of over-dramatization.
So many shows today--both true crime and paranormal shows, and UM was both--feel that they need to make everything as dramatic as possible to hold the audience's interest. Unsolved Mysteries wasn't this way, though.
Instead of hyping everything up, UM had a very low-key, straightforward, old-fashioned documentary approach to the way it told stories and I think this is a style that is desperately needed in modern programs. In fact, I'm pretty surprised that no one has come along and just completely ripped off the Unsolved Mysteries formula. If UM is not going to produce anymore episodes, then someone else should come along and take the torch as the show's spiritual successor!