I have a tendency to see the flaws in TV shows, so that if even if I start out enjoying them, my enthusiasm not only fades, it turns into more or less contempt.
Also, I HATE HATE HATE HATE HATE (etc) commercials, so I only watch shows on DVD or downloaded via torrent.
Firefly was awesome. It's kinda bittersweet that it only lasted one season, because they set the bar so high for themselves that I could see myself falling into the pattern I mention above. But to contrast, Joss Whedon's writing for Buffy I found just sophomoric - I laughed, but I wasn't captivated like I was by the characters, dialogue, settings, etc. of Firefly.
Heroes has turned on me. How did Mr. Petrelli get back in time to steal the catalyst from Hiro before he actually stole Hiro's time-traveling ability? And how did Ando, Hiro & the girl get forward in time once they'd traveled back? I'm totally willing to suspend certain amounts of belief for the sake of things (such as using faster than light speeds to travel back in time), but they've violated the continuity of their own paradigm a few too many times.
Lost ... well, I'm looking forward to the new season, but that series has done a few things that rather than make me anxiously await answers to the mystery has instead made me feel like the twists and turns are becoming overhyped, and the actual meat of the story is ailing.
Dr. Who is just plain silly, and as long as I look at it as a comedy with spacey eye candy and not as Science Fiction, I enjoy it.
I used to really enjoy Futurama, Family Guy, South Park, The Daily Show, The Colbert Report, Clean Sweep, and Neat ... but I never exert enough discipline to keep up with them, so it's been a long time since I watched any of those.
I caught a couple of episodes of House, and thought it was entertaining, but only mildly.
I caught two partial episodes of scrubs, and thought it was deplorable. Any of the jokes made were so contrived and poorly acted that I felt like I was being insulted.