Fans seem to want their favorite shows to be love affairs that last forever and a day... I would rather a good show have a short run than see it extend beyond its natural life into a parody or pale imitation of itself.
I know all the reasons why it will never happen, but I'd like to see a move towards the way some British shows (like Prime Suspect, for example) are produced and marketed:
- Conceive each season as a stand alone story. Don't leave major plot threads hanging. Get rid of the season-ending cliff-hanger.
- Don't try to turn a show into a brand name: rather, focus on actors and writers.
No show can make the "never-ending storyline" thing work, so it's better for a show's creators to end things on their own terms than to be forced to wrap things up due to fading ratings.
Long running shows that are successful tend (a) to be completely episodic with no real serial elements (Law & Order, although even there the formula is showing its age) or (b) to keep the central situation the same while shuffling new characters in and out (like in E.R. and, to a lesser extent, The Wire, which is one of the only shows I can think of that has consistently gotten better each season).
The creators of shows like Lost, Battlestar Galactica, and 24 would have been much better off if they realized that the limited nature of these shows' central premises almost required that they be resolved at the end of the first season. I almost thought Battlestar Galactica had done this - that the third season would mark a complete change in the show, moving from outer-space chase to planet-bound life-under-occupation drama - but after a few episodes they were back in space, doing the same old, same old, but falling into all the traps they were smart enough to avoid the first time around.
(Plus, I agree with something he brings up in the comments: one of the reasons I like Freaks and Geeks is that it said all it had to say and then had the dignity to get cancelled.)