I enjoyed this episode a lot.
But did anyone else think that this could easily have been re-written to be the 50th anniversary ep or a multi-Doctor one? The warping of time inside the TARDIS could have allowed 11 to bump into his predecessors, with some technobabble explanation about how time was being re-written and they hadn't regenerated, explaining away their appearances.
I was a little surprised that this ep didn't contain a few more references to the past; maybe some props from previous Doctors (hats, scarves, umbrellas etc) or even something to acknowledge past companions turning up in the TARDIS. Just little Easter eggs for the fans, really.