The British cast is likeable, have good chemistry and somehow the script works. There's something about American producers taking a good idea and putting it into the hands of actors who aren't likeable, have no chemistry and make it as boring as hell. Coupling would have done better if they had toned it down and not used the original source material. When you adapt a foreign show, its NOT necessary to repeat the original material word for word. It shows not only a lack of creativity but also a contempt for the intelligence of your viewing audience.
I can sorta kinda see their reasoning for using the word-for-word British scripts; it allowed them to hit the ground running with "proven" material, while giving the US writers and producers a lot of time to figure out where to take the American characters next season. (Had the show been successful, NBC would have ordered anywhere from 18 to 24 episodes this season alone, using up all the original scripts.) The fatal flaw with this plan is it allowed newspaper TV critics, practically all of whom were familiar with and enamored of the original UK version, to see just how disastrously miscast and badly produced produced the American version is, because they were able to directly compare episodes. (To a lesser extent, the same thing happened with word-of-mouth amongst the viewer demographic NBC is most coveting, many of whom themselves watched Coupling on the BBC. Not only did they tune in and find that the NBC version was crap, they found out that they'd already seen the exact same show done better a year ago. So word spread...)
Basically, NBC wanted a formula to replace Friends, and they got what they were seeking. When you try to design a show from the ground up to fit a certain specification instead of simply let the creative process happen, you end up with a bad show.