I agree JapaneseMargaret, just because something is British/American/whatever doesn't make it good/bad. It's down to personal taste as well as the quality of the writing/acting.
I for one found Friends hilarious and never missed an episode, and now, if it's on, I will watch it and laugh at it, because I still find it funny.
For me, a good sitcom is about the characters first, then the situation. With Frasier, for example, which I find hilarious, it's basically about social awkwardness, not psychiatry.
Big Bang Theory is, if you think about it, Friends and Frasier combined.
I agree that the formulaic British sitcoms aren't as good, but the slightly more left field ones, like the IT Crowd, Black Books, Father Ted (admittedly Irish but all written by the same people) are still laugh out loud funny, and things like Blackadder and the Fast Show will never get old.
The nostalgia element doesn't do it much for me, it just makes me feel old
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Talking of which, I stumbled across "Melissa and Joey" yesterday. It "stars" two massive kids TV stars from the 90s when I was growing up, "Sabrina The Teenage Witch" and Joey from "Blossom". God, it's crap. And it's a current show, they still make it, although it wasn't overly obvious why.....