My TV diet for the past few weeks has been restricted to test cricket (the Sri Lanka series wasn't at all funny for England supporters) and two American sitcoms: Frasier and 3rd Rock From the Sun. I also watch reruns of the British comedy Coupling when I remember.
The big difference between American and British sitcoms is that the American ones are optimistic while the English ones are melancholy.
American comedies poke fun at the foibles of successful people. The eponymous Frasier Crane and his brother Niles, for all their absurd affections, are cultured professionals who pursue happiness and to a large extent achieve it. The aliens of 3rd Rock usually get their own way on earth despite their inadvertently outrageous manners. Though I didn't particularly care for Friends, they were hardly inadequates. According to Wikipedia, some people think Seinfeld is nihilistic, but I find it just cheerfully callous, and I love the inexorable logic of the way each episode develops a trivial event into an absurd crisis.
Most of the funniest British comedies, though, are about failures. The manager in the British version of The Office is so ghastly, and his victims so helpless, that I couldn't bear to watch more than a few episodes when it first came out. Victor Meldrew of One Foot In the Grave is an embittered old man whose character flaws lead him into hilarious predicaments with depressing resolutions. All the characters in Only Fools and Horses are small-time conmen whose scams generally go wrong. Count Arthur Strong could be regarded as senile (I have to say I thought he was just eccentric and absent-minded when I wrote this post; senile isn't so funny).