The difference between British and American comedy is that:
In British humour surrealism is a main factor. The weirdest, most random type of humour is genrally funny in a British sense, and it's viewed as intelligent. But when an American tries surreal or random humour, it fails and i'd be viewed as dumb and stupid.
That's why Britain has weird sketch shows like Big Train, Mitchell & Webb, Jam and Monty Python while America has normally funny sketch shows like Saturday Night Live.
But you missed Mr Show, Adult Swim shows, and I'm too lazy to think anymore....
Yes actually the defining factor in British sitcoms/sketch comedy is sadness/pathos. The humor is generally rooted in situations that, on reflection, are really quite depressing. American Sitcoms on the other hand are generally happy at their base.
The whole surrealism thing has more to do with the fact that many of the comedians of the '60s were influenced by absurdist theatre and they in turn influenced later sketch writers(both in England and in the US and Canada). And actually if you look at the work of Rik Mayall and Adrian Edmondson you can see a pretty obvious influence of Beckett on their work (especially in
Bottom)
I realize that the above qualifies me for a place in
Private Eye's psueds' corner, but sod it it's accurate even if it's all high falutin' sounding.
So to illustrate, here's a wonderful study in pure patheticness from Steve Coogan:
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