I more or less agree with Ross Douthat's general point in this post on why, in a lot of cases, with good TV shows, shorter runs are better than longer ones.
My slight disagreement has to do with The Sopranos: while I think it could have used some trimming, a lot of what makes it a great show (as I have suggested) is also what makes it a bit of a mess. Serial TV shows (or comic books) are different from movies in this respect: they sometimes work best when they have room to grow, to make wrong turns, to build what were originally minor ideas/incidents/characters into major elements of the show. That is, there's the opportunity for a kind of narrative improvisation that happens on the writing/production level.
(Matthew Yglesias responds to Ross's point with the comment that shows require "planning". I more or less disagree with this, or, at least, I think that having an overall structure that is (at least partially) open is one of the strengths TV serials have over films and novels)*.
So, I'd adjust what Ross is saying, just slightly: rather than have hard constraints on shows ("David Chase should have told his story in four seasons"), I'd like to promote the idea that shows don't have to go on forever. So, if you're a fan of, say, Firefly/Serenity (like I am), don't whine that you aren't getting any more episodes/movies: be glad that what we have is pretty darn good.
Also, Ross asks "would My So-Called Life be remembered as fondly as it is if we'd had to watch Angela Davis and Jordan Catalano get together, break up, get back together, break up - and then, worst of all, go off to the same college?" - which is exactly the problem with Veronica Mars right now.
*There's a difference between (a) coming up with a story that will take 30 hours to tell and then breaking it up into forty 44-minute pieces story line and (b) coming up with 6 hours worth of story and then figuring out the next 6 hours based on things like which elements resonated most with the writers, producers, actors, audience etc. Both approaches can result in "good TV", but approach (b) is something that TV serials can do really well.