It had some nice "human" moments, the gathered group was a little wasted--a problem whenever you have too many people in a story--why you end up with J'onn J'onzz staying on the space station all the time in the Justice League, or groups being isolated in stories. Someone always ends up twiddling their thumbs. When they (the writers) do manage to really use everyone's talents well--well, it's very exciting.
It posed the interesting problem of which is worse, to have traveled with the Doctor and not to be traveling with him any more or to have traveled with him and not be able to remember it--to have saved the universe and have to go back to being the sad loser you were before the Doctor made you better. Rather like the question that comes up repeatedly in Doctor Who circles, which is better--the world we have with our Doctor Who adventures, or a world with no stories, but a real Doctor. Perhaps we have both but how would we know? I would almost call this "The Flowers for Algernon Problem." Donna will never know what she has lost, except in the sad looks her grandfather gives her.
As to the bit I didn't like--am I the only one who never wanted to have sex in the Tardis? Who thinks that the Doctor should be asexual--beyond petty human desire? Of course in the early days it was easy--I doubt anyone was lusting after William Hartnell, Patrick Troughton or Jon Pertwee, but it got a little strange with Tom Baker (partially because he, the actor, in fact married his costar) and then Peter Davison was young and attractive, but never to me. I suppose I had a little trouble with how hot I thought Paul McGann was, but again I never wanted that to intrude into the show. It made Human Nature that more poignant--that the Doctor had to become human to feel emotions. But I always liked Spock and Data too--fully functional, but emotionally isolated.
Ah, Doctor Who. It can go to A-Z, the biggest of them all, world's destroyed and back to A in another time and another place. In fact, A is constantly changing. I mean, it started life as a sort of soft history show for children with the curmudgeonly grandfather and his granddaughter and just kept evolving.
The problem that that constantly moving A can bring is the Superman problem. When you have some one who is well nigh indestructible with super powers--how do you make the outcome uncertain? Who can beat Superman? Other Superheroes. Who can beat the Doctor? He can otuthink anyone, he'll regenerate if you kill him, etc. And then he's an alien (and they did fall into this trap). There's always some "previously" unknown physical quirk or circumstance that can save him (in this the residual regenerational energy channeled into the hand that he lost the last time he regenerated that was touched by Donna creating a duplicate but human Doctor--got it?). But aside from that, well done.