The Secret of Cheers, or How to Maintain High Quality for a Really Long Time:
Beliefs about a current renaissance of TV quality notwithstanding, I have occasionally pointed out that network shows are at an extreme disadvantage compared to cable because their runs have to be longer: 22 or 24 episodes rather than 6 or 10 or 13. This is also true of the length of shows themselves: 25 minutes in the 80s for a 30 minute show, rather than 22 (or less; Archer runs just barely over 20) now. Given that, I find it remarkable that Cheers is easily and obviously a better sitcom than anything else that has aired since: more time at a higher level than anyone else. Now, like most viewers, I don't like them all equally (I could ignore most Cliff episodes and not feel much loss), but the quality is always present.
The secret, as it turns out, is in the parenthetical above: there are a few episodes each season devoted to specific characters: 25 episodes might include...
2 Norm episodes (or 3)
2 Frasiers (after season 3)
...a couple that introduce random characters into the mix, and a couple that focus on aspects of Sam or Diane/Rebecca's personality that have nothing to do with the main story arc. So whatever that story arc is, it only has to drive 10-15 episodes in a season, sometimes fewer: you've just managed to create a smaller prestige show inside a larger network one. Further, the rules are different in the non-arcing episodes: Norm and Cliff have to end up pretty much where they start, and to the extent anyone else gets an arc, it sets up slowly over seasons; each episode is mostly an excuse for the writers to do a funny idea. It hardly matters whether it goes anywhere or not, since that's not the viewer's expectation.
Compare this to other shows, including those that are explicitly attempting to work within the Cheers paradigm, and it's clear how hard this is to do: Parks and Rec can hardly do a story without a significant Leslie component even if it's "about" someone else (this, I think, is why the Ron and Tammy segments are so well regarded), or how much New Girl flails around with how central Nick and Jess' relationship needs to be.