In his blog entry ("The Secret of Success", 10.02.2005) about American sit-coms, Yahtzee says the reason they suck is breaking a set of rules that British ones follow precisely. I might agree with half of what he said, but the other half is bullshit. I suspect it's due to him having a very particular taste and being quite British.
Firstly, Friends != Horrible Shit. Sure, it has kind of turned into a soap opera, but almost every fucking episode IS funny. Compliance with an arbitrary set of rules does not define the funniness of something. Something is simply funny or not, although that can be quite subjective. The quality of the humour in Friends is way above that of the standard American sit-com. To see that, there's no need to analyse it's success or break it down into a set of rules -- it should be obvious to anyone who has watched even a few episodes.
Secondly, Seinfeld is way above Not Too Shabby. You can simply tell that from the various contexts it is often referenced in. The jokes are often quite unique, compared to other sit-coms. Or even if they aren't, they sure fooled me. For example, google for "Costanza principle" and download the powerpoint file "Strangling Legacy Code", then search for "Costanza" in the file again. It shows how a joke from one the episodes can apply to solving a real-life software engineering problem.
The others that Yahtzee mentioned I pretty much agree with. But my point is that there are more kinds of comedy than the British variety, and it can be equally good. Usually I prefer British humour as well, though.
The one thing about sit-coms (American or not) that bothers me is the laugh track. What the fuck is that good for? I don't need to be told if something is funny or not, I can figure it out by myself. The only sensible reason for the laugh track I can think of would follow this logic. They say laughter is both healthy and contaiguous, therefore more laughter should mean a healthier population (which would be good, if the Earth wasn't overpopulated).