In writing, we rarely talk about a great book without reference to character and how the main character grew and changed over the course of the book. It's that character arc that takes a book from good to great. At some point the character reaches their end point, they have grown enough to have earned their happy ending and as a reader we close the book with a sense of satisfaction.
TV shows that focus on one character, or even a few, do the same thing, usually over a season, or two. But after three seasons, with the characters still making mistakes, and not growing quickly enough, especially in character driven shows like House, or Grey's Anatomy, I get frustrated.
I'm there right now with 30 Rock, a show I still watch every week, and love, but no longer find as funny as I once did. It's starting to feel like a 'been there, done that." Lemon still has self-confidence issues, Tracey is still an idiot, Jenna completely self absorbed and there's not much new in that.
House I gave up on several seasons ago.
Everything else I'm watching is either at the three seasons, or less. There are some shows that are exempt from the rule, like procedurals where the focus in on the crime of the week, or something like Game Of Thrones where the huge cast of characters and complex plot keep everything moving.
I'm even concerned about my beloved Vampire Diaries. Will I ever get tired of Damon's snark, or Alaric's morality or the brother's competing for Elena's attention? I'd rather it have three seasons than get tired in my memory and lose what I love about the show.