The main thing that differentiates American sitcoms from every other country's sitcoms, as far as I can see, is that there always has to be a moral lesson somewhere in the episode. If you don't realise how unusual this is, try comparing a Canadian or British sitcom to any American sitcom of your choice, and you'll see what I mean: while some non-American sitcoms do involve moral messages (Corner Gas is pretty heavy on "don't underestimate people just because they're poor/uneducated/not from Toronto"), it's generally pretty understated and also isn't necessarily a requirement for every episode, whereas even Frasier is given to delivering explicit homilies right after the climactic scene.
Now, by its very premise My Name is Earl is set up explicitly as a series dealing with moral lessons. After all, its titular character is a small-time con-artist who has reformed and is trying to make up for his crimes, and each episode revolves around him atoning for some past crime and usually learning some sort of related lesson-- gay people are human too, paying taxes supports public services, stand up to big business, own up to your mistakes, etc.-- so you'd think it would be the comedic equivalent of a Baptist Sunday School class (and one from the sort of Baptist church that frowns upon gospel singing and Getting The Spirit). However, the very fact that the central characters are deeply amoral allows them to get away with it: when Earl makes some kind of climactic speech about how he's just learned he should be nicer to his brother, you can genuinely believe that it's only just now occurred to him that there is value in doing this, and the series' simple the-good-do-well-and-the-bad-suffer setup also allows the series to deliver rewards and punishments to its characters without incurring a "c'mon, that's unrealistic" reaction-- if anything, having had the setup, you're just waiting for the moral payoff.
I compared it to M*A*S*H above, and it occurs to me that the earlier sitcom also managed to get away with its periodic moral lessons for precisely the same reason. Everybody in the surgical unit was as morally bankrupt as the characters in Earl (with the arguable exception of Radar, but he had the innocent-who-does-as-much-harm-as-good thing going, which leads to similar results), and so you could actually believe in them, if not learning from their misdeeds, at least providing a non-preachy example to others through them. So
Now, if they'd only manage to pull off this trick with Will and Grace, maybe I'd actually be able to watch more than five minutes before switching off in disgust.