HertogJan: Some of the shows mentioned in this thread, started out good or became good after some episodes. Shows like The X-Files, The Office US, Dexter, ...
The problem is they became very popular and as a result dragged on for too long resulting in characters, story lines etc. becoming uninteresting, unrealistic, etc.
While I agree with you about most of those shows dragging on for too long. X-Files never dragged on for too long. I remember mourning that the show ended, but was excited for the movies that were promised. We got what two movies, not enough in my opinion. Look at the two of them now, Duchovny has a Soft-core porn show on Showtime or Skinamax and no one has heard from Gillian since.