You need to jump back in. This season is an avalanche, picking up steam every week. Last night I damn near peed my pants.
You need to get back in for the final season. Harrowing.
I came thisclose to pulling the plug during Season 2, but I am sooooo glad I didn’t. The show has since developed a wicked sense of humor, and has lightened up on the constant angst.
BSG. I was really into it <still am> but when they started this ‘here’s a couple episodes, and we’ll give you a couple more next year’ bit, I quit watching until they were all done. I’m currently in Season 3, watching them when I want to damnit.
House has already been mentioned, but I do like the ending episodes. Two others that you guys probably wouldn’t think of was “At the Movies” when they changed all the hosts, and, similarly, “The Man Show.”
Of the ones y’all mentioned: I didn’t get far enough into “Dead Like Me.” I’m at 4th season of Smallville and no slowing down (it helps that I’ve read a lot about the myths. I’m very interested in the Phantom Zone and the Superman mythos in general). Oh, and Lois is probably the best actress on that show. She actually talks, instead of just staring at people while the camera moves.
Anyways, I’ve not really given up on Glee, but I did get behind. And Grey’s Anatomy, while interesting in a soap opera way, was not a favorite. The rest I’ve never seen.
Most things really. I find that as soon as I can miss one episode and not care that I missed it I’m done. Many of the things mention qualify but most I give up on halfway through season one. Examples are Heroes, Weeds, Ugly Betty, Madmen and Damages, each of which I heartily recommended to friends but lost interest in before the end of the first season.
Another for House. I just don’t care about these miserable people and their emotional constipation anymore.
Both “Soap” and “Desperate Housewives” in the middle of the third season. Over the top parody lasts only so long.
Weeds: Essentially dropped this like the turd it was because the main character gets the DEA(?) agent killed because he bitch slaps, deservedly, her whiny son.
BSG. The last season: It is supposed to be science fiction, not metaphysical crap where everyone turns into the enemy.
Lost: I can’t remember which season, but unless you actually explain something I ain’t watching anymore.
Survivor: I’ve watched a grand total of 5 minutes of that show. I felt my brain being sucked out of my sphincter in just that short of a period. No thanks.
Alton Brown’s Caribbean adventure. I don’t even care enough to look up the name. The same damn food, over and over, while he tooled around in a catamaran that probably costs $1M.
That was my experience as well, but I don’t think qualifies as a response to the OP. There is no way in hell I would give that show an “impassioned” half a season before bailing.
Heroes, Lost , Friends and 24
I hated House as a drama, but then I realized it was really a comedy and I’m totally into it again, its hilariousness, pure comedy genius. 
Going back a ways, I used to love NYPD Blue. Never missed it. But then it got so ridiculous I began to think that my TV relationship with Sipowicz might get me killed IRL.
More recently, Rescue Me. Another never miss show, but I found myself not watching them last season as they backed up on the DVR, then eventually deleting them.