This was awesome. It was mostly continuity porn, but that's kind of what you want from an anniversary special, and it tied so many threads together so well. It picked up threads from the Davies era, fleshed them out more fully, and brought them to a very satisfying climax.
And there were so many wonderful moments, including a lot of the interactions among Doctors. Hurt got some great zingers. "What are you going to do, assemble a cabinet at them?" "I hope the ears are less prominent this time." And I loved the bit where he wondered why his future selves were so afraid of acting like grownups, and they looked at him.
And some very clever timey-wimey stuff. ("Honestly, I don't know where he gets these things from," Ten says defensively.) I loved the idea of starting a program in one sonic screwdriver and getting its results in a later incarnation (and it's kind of a nice bit of symbolism that the screwdrivers are "regenerations" of the same screwdriver). I was a bit frustrated when Clara got them out and we were deprived of the payoff, but that turned out to be because the whole thing was setup for a much greater payoff at the climax. I love that kind of deft seeding of things that you have no idea are meant as setup for something later until the payoff comes and you go "Ohhhhhhhhhhhhh!!!!" The bit with the stone dust and the draped statues was another one. I caught onto that just seconds before Dr. Osgood did.
Although I predicted a setup/reveal that didn't happen -- I kept expecting that the painting would turn out to be the pocket universe where Gallifrey was hidden. But I guess Moffat wants to set up a "The Search for Gallifrey" story arc instead.
And so many Easter eggs. Clara teaching at Coal Hill School, and the sign saying I. Chesterton was the headmaster or something. The blackboard in the Black Archive with all the photos of past companions and associates (did I see a photo of Brigadier Bambera?). The round things (which were properly called roundels) -- I wonder if they borrowed some set pieces from An Adventure in Space and Time for that scene. Moffat even picked up on the hints from the RTD era about Tennant and Elizabeth I having a thing and fleshed them out. And one of my favorites was when Bad Wolf described the TARDIS sound as "that peculiar wheezing, groaning sound" -- which was how Terrance Dicks habitually described it in so many of the novelizations he wrote!
We even got an explanation -- well, a passing handwave -- for why the Doctor doesn't remember the details of his meetings with himself. It's because "the timelines are out of sync" and the extratemporal memories aren't retained. I should've known Moffat would address that (although he's created a continuity problem within his own body of work, since Ten did remember the events of "Time Crash" from Five's POV).
No surprise, also, that he filled in another gap by showing the beginning of the regeneration from Hurt into Eccleston. Up until that shot cut away, I was hoping we'd get a surprise cameo by Eccleston, but I wasn't surprised when we didn't.
But man, the surprise cameo that came right after that more than made up for it. "Revisit some old favorites" indeed.
It was also very handy that the anniversary/crossover came at a time just before Smith leaves, so that the next Doctor could make a cameo. So I guess now it's official -- Smith is actually the Doctor's twelfth life and Capaldi his thirteenth. Which means, given Moffat's demonstrated devotion to continuity, that the upcoming Christmas special may well explain the origins of the Valeyard.
Now, the question is: Did the Doctor actually change his own past, or is this the way it happened all along and he just didn't remember? I think it was the latter. After all, the Moment was the source of the temporal crossover, and the Moment was there in the "original" history, so there's no reason why it would've unfolded differently than what we saw. So it hasn't actually changed the Doctor's past, just revealed something he hadn't known about it before. But that revelation is likely to change him, to lift a weight from his conscience. Maybe he won't be so afraid to act like a grownup anymore, and maybe that's why his next incarnation is older.
(I wonder how many other comments have been added since I started this post...) (EDIT: Wow -- 41!)