a) They have interesting, well-written characters.
True Blood and Glee are the most recent examples. But in the past I've also loved watching Grey's Anatomy and Desperate Housewives (my guilty pleasures).
b)They have an interesting concept or idea
Again True Blood comes to mind. One show that succeeded at this but after a few seasons later, eventually failed, was Lost. Season One for that show was so gripping, I had to cancel or postpone evrything that I had to do on the night it's on. But after a while it became to complicated, I got well...umm..lost. I'm very impatient and it was bound to happen sooner or later. If they got out of the island after the first few seasons and became a series about the aftermath of the tragedy and how it affected everyone involved, then yeah I would have kept watching. But no, they stayed on that island then I just became bored.
Right now, I'm following True Blood. Instead of waiting for the series to come on TV (which hasn't happened yet here in Australia) I decided to just purchase the DVD set, which I admit has an incredibly cool cover art. So I'll probably write a review soon about the first season, when I finish of course.
Glee does not show here in Australia until September. Sad face. But they had an advance screening last night and I can't wait for the series!