I think British comedy is often a bit more understated than American comedy, something which doesn't always translate well to an American audience. I generally find with American comedy that characters tend be larger than life and unbelievable, whereas in British comedy they tend to be people I can actually sympathise with. I found that to be the case with the US Office.
Very true. When I watched it, I got the feeling that I was merely spectating people during a regular day at, well, "The Office". I expected more obvious jokes and maybe some slapstick like in Garth Marenghi's Darkplace (That show is hilarious, by the way).