I have only seen very little of both, but I did get the distinct feeling that the US office was just plain better executed in many ways. I do remember that reading online forums, fans of UK Office scoffed at the US version at first, but that turned around.

True, after the first season. But I kind of question whether there is really a difference between "better executed" and the cultural difference we're discussing here. It's rare to have a show change tone from one season to the next, so it gives us a pretty unique way to look at what changed. I'm not sure what the "execution" issues were in the first season, except that it seemed (to me, as an American) more cold and self deprecating, where later it was warmer and more lovable. It had the same actors and sets and everything, but just different writing and changes to the personalities and storylines of the characters. But I think this might just be restating the differences between American and British humor that kicked this thread off?