For anyone whos been watching Doctor Who on BBC America there are, I believe, minor cuts that are made in the American broadcasts (do we still use the word broadcasts for cable? or is it transmissions?) by comparison with the British ones; but for this seasons opener, The Eleventh Hour, there were major cuts, because in Britain the episode ran twenty minutes longer than a regular episode but the American broadcast/transmission squeezed it into the same time frame as all the others. So if youve only seen The Eleventh Hour on BBC America, youve missed at least twenty minutes of material, and you owe it to yourself to track the episode down online (the Dailymotion version, for example, is complete).
Some of the cut scenes are not only quite good in themselves, but also seem to be setting up important aspects of the season arc; a number of important moments in later episodes (in particular Flesh and Stone) make a lot more sense if one has seen the cut scenes.