I think that the worst thing for many television shows is plot progression.
Take The Office (US Version). Season 1 was more or less a replica of the UK version. In Season two it really came into its own. I think Season two of that show was one of the finest television achievements of all time.
Then it got worse. The writers couldn’t help themselves. They had to progress ‘the plot’. The characters changed, the relationships changed. The finely tuned balance was disrupted.
And it stopped being funny.
I love the second season. I miss the second season. Seinfeld, The Simpsons, Arrested Development, 30 Rock, Family Guy. These shows were funny to the extent that they DIDN’T change once they figured out what worked. If they did and realized it sucked, they found a way back.
Comedy is a formula, not a narrative. Once you get a mix of characters and relationships that work, don’t screw around with it.