Well I was hooked on Dare Devil, The IT Crowd, House of Cards and Firefly on the first episode, but these shows pander to me beyond just plot, I'm sure you can detect the theme for most of them. I'm sure there is a key point for most shows that most who watch up to that point end up watching the whole thing. BSG I remember took 10-12 episodes before I really got into it, and I only went that far because people wouldn't shut up about how good it was. There are also shows I really liked but either dragged on or I just lost interest in or was just frustrated by the characters (The Killing, Buffy, etc)