I'll agree that the Utopia bits turned out (apparently) to be a disappointing throwaway and the absurdity of the year 100 trillion thus presented, but then, the ostensible story's not always the important thing : compare to School Reunion. It brings together several different threads established in season3 - I think they've finally got their act together about how to do a "big bad" in Doctor Who: the attempts in season1 and 2 were rather anticlmatic. And, hey, not much worthwhile happens in episode 1 of Logopolis, either! ;)