I think in general a lot of the more successful British comedy is more 'intelligent' comedy- irony, satire, clever play on words etc. Whereas the comedy I've seen is using stupid voices and yo' mamma jokes... I guess the comics have to aim at the average IQ of their viewers which in the states equates to that of the average British PE teacher.
Edit: Actually that probably relates more to stand up. They have certainly put out some good stuff with TV etc- the obvious one that sticks in my head being Family Guy which is kind of stupid humour (baby wants to kill mum, fart jokes, drunk jokes) but with some fairly off the wall direct hits using satire.