Since I've been an adult, I can pretty much name the TV shows that I've watched through their first run on the fingers of one hand. Frasier, Modern Family, and The Walking Dead are the top three. I came to Nip/Tuck in the second season but was hooked then. But even Star Trek: The Next Generation I caught mostly in reruns, though that had more to do with the fact that I was very busy with my first teaching job than from anything else.
I came to The Office and Big Bang Theory several seasons in, but did make the new episodes part of my regular viewing once I'd been hooked on them. Three other series that I started on the first episode with were Flash Forward, Falling Skies, and Terra Nova. I was very disappointed when Flash Forward was cancelled, but I quit Falling Skies pretty quickly and was ready to give up on Terra Nova when it was cancelled.
I'm now watching a new series and have to admit I'm still hooked. This is The Following, with Kevin Bacon as an ex-FBI agent who gets involved with the wife of a serial killer and then becomes the target for an elaborate revenge plot by said killer. I think it's Kevin Bacon's acting that is keeping me hooked. He's very fine in the role. At least it seems to me.
It's also the acting of the ensemble casts of Modern Family, Frasier, Big Bang Theory, and The Office that kept/keeps me coming back to those shows. Nip/Tuck was about the totally insane plot twists that they got up to in that show. And The Walking Dead is a combination of acting and plot for me.
How about you? What shows are you truly hooked on? And is it the acting, the stories, or something else that keeps you watching?