It's the right time for David Tennant to leave. Three seasons (and a bit) is exactly the right amount for one Doctor, even if he is immensely popular. Any longer and he'd just be repeating himself. He's done it all. There's no more room for that Doctor. The last twenty minutes of 'The End of Time' brilliantly summarised his time on the show, as he visited all the people he thinks he might miss (including the descendant of John Smith's wife from 'The Family of Blood'). David Tennant has made it look easy - so easy that you you don't really notice that he's spent years building this character. The time has gone quickly because he's never been short of excellent, even in the slightly average episodes.
But of equal importance is Russell T Davies' departure from the show. He managed to take a franchise that I didn't care about (because I'm not 40) and created one of those rare shows that is mainstream and nerdy at the same time. It's difficult to make four seasons of anything, and he's managed to keep the quality up despite significant cast changes in each one. It's been the same show but always noticably different. It'll be interesting to see whether the tone changes with Steven Moffat. It's unrealistic to expect every episode to be 'The Girl in the Fire Place', because he has a responsibilty to the time slot, but maybe there'll be less running and silly aliens. Probably not. One thing that is obvious though, is that we won't be seeing any of these old characters again. Rose, Martha, Mickey, Donna - they're all done with now. Time to start again.