Unlike Laverne & Shirley and Happy Days, Mork & Mindy had already ended by the time this show premiered. So in a sense, this can be viewed as a continuation.
Besides Fred McConnell and Orson (Played once again respectively by Conrad Janis and Ralph James), Eugene also returns. He only appeared in the first season of Mork & Mindy, and is played by a different actor this time.
Although Robin Williams did voice Mork in a couple of episodes (earning a "guest appearance" end credit), for the bulk of the series the character was performed by Dave Coulier.