2. Special guest stars

You can't begrudge struggling shows for trying to lure in new viewers with a celebrity guest star. But when you're throwing some celebrity with a huge ego and little to no TV experience into the middle of a tight-knit cast of actors who usually play off each other brilliantly, the results are bound to be unsatisfying, especially for die-hard fans. "30 Rock" is a repeat offender, but the worst was Jerry Seinfeld stopping by to drop lots of references to that bee movie.