Guest Who?

As someone whose Netflix queue is 95% television, I spend plenty of time thinking about ways to make my favorite shows even better. I'm almost always far more interested in character development than any other aspect of a TV program. I'll keep watching long after the plot has gone off the rails so long as my favorite characters remain consistent -- and consistently entertaining.
Case in point: I sat through the entire second season of Heroes, which was one of the worst sophomore slumps I've ever seen, solely because of Zachary Quinto's gleeful, hammy psychopath Sylar. On the other hand, I've pretty much sworn off How I Met Your Mother, formerly one of my favorite shows, because of the way its writers have destroyed characters' continuity and likeability in the last few seasons.

A crucial factor in the success or failure of a show's character development is the integration of its guest stars. Some shows get it right (The Big Bang Theory comes to mind, with Wil Wheaton playing himself as a hilarious nemesis for Sheldon) and some, well, don't (ahem, How I Met Your Mother, with stunt casting choices like a wooden Britney Spears). Famous guest stars draw ratings; they can also draw the audience out of a necessary suspension of disbelief. But if it's done right, the guest star fits into the cast perfectly (Jon Hamm on 30 Rock, anyone?) and makes you wish the role were permanent.