Thanks to the mighty powers of the internet(protocol) I started to watch quite a few TV shows which I could never watch without the help of the aforementioned powers. (Because a) I don't have a TV and even if I did the german-speaking TV station(s) (I could receive) completely fuck up everything when broadcasting TV series: most of the time they only broadcast the dubbed version (argh), they drop episodes (because of some stupid sports event that HAS to be broadcasted...), the mix up the correct order, ..)
Anyway, the point I'm trying to make (but haven't started making yet) is independent of the broadcast mechanism.
Some of the shows I recently enjoyed are Dexter, The Big Bang Theory, Chuck, United States of Tara, and even (to a lesser extend) Weeds, House M.D., Grey's Anatomy and Desperate Housewives.
All of them started great. They were funny, suspenseful, or both. They had interesting plots, of course written for maximum WTFness (how often do you encounter any of this in the real world: serial killer working for the police, schizophrenic moms, hard-core nerds trying to date hot actresses, geeks turned superspys, genius but complete asshole doctors, hospital stuff who constantly switch partners until everyone has slept with anyone...).
During the first season, you get to know the main characters, the various support characters, and there is some halfway believable plot stretching through the whole season (which by the way discerns a TV series (where each episode has a distinct plot, e.g. your classic cop series or CSI) from a TV serial (which I prefere to watch..)).
The second seasons might be better than the first (Dexter), or slightly worse (Chuck). But most things still make sense, the characters are still behaving mostly like normal people do (though normal people with little common sense), and we don't have to cringe too often when the make obviously wrong choices.
But at the latest starting with the third seasons, things go downhill. The writers have to jump through hoops to make the basic premise of the show hold up. They cannot allow any real change, because that would destroy the unique selling proposition of the show. The relationships between the characters cannot reach a stable state (which basically is what they all (we all) strive for). Things just become ridiculous. But because the show is a great success by now, they continue to produce more an more seasons until you hate all the characters for being so incredible stupid, for never getting out of their misery, for never resolving the problem that made them interesting to watch in the first season.
Please let Leonard get Penny and kick out Sheldon. Please let Dexter live happily ever after with Rita and the kids. Please let Chuck get rid of the Intersec but not of Sarah (actually, he should get rid of her too and reunite with the owner if this deli), and let them start a Perl based startup. Please! Pleeeeeaseeee!!??!?
PS: The only shows that can go on (nearly) forever are family based sitcoms (Roseanne, Malcom in the Middle), which are forced to change because the (kid) actors grow up and thus have different problems (plus, they're not based on some crazy basic idea). Or cartoons like the Simpsons. Or crime shows.