The secret to a good Sitcom is not dragging it out and having a smurfton of episodes. Brevity is the soul of wit. It's like, yeah, Ray's mother is still pushy and his brother is a big mopey whiner, Sheldon is still a dick, Ted is still a lonely dork etc. etc. We get it. We've heard this joke dozens of times. Not funny anymore. It's always interested me that American shows can have 20+ episodes in a season, whereas British ones have often been traditionally six, and on the whole I find British sitcoms to be funnier than American ones. Having said that, Fresh Prince lasted ages and was still funny, and It's Always Sunny is hanging on in there!
I think as well, when you start getting all hokey and making the focus be the events the character's lives (Obligatory Wedding Episode and Baby Story Arc are not and never will be funny) rather than funnies you're on shaky ground and bordering on soap opera territory.