I nearly lost it when Hartnell said "I don't want to go". Pretty sure Gatiss intentionally mirrored Tennant's line from The End of Time there.
That got to me as well--along with Smith showing up, in that enigmatic way. Why didn't they show all the rest--well they did that with "Day of" but maybe, like Smith holding his own with Tennant, send up Matt's incarnation as one of the best--after all, he copied Patrick's most of all--and that was Hartnell's immediate successor.
having Hartnell actually fix the time rotor was my favorite bit.
This is Mark Gatiss' best offering in my opinion--and I would like to see more--maybe a behind the scenes tale of each Doctor--with that doctor's actor as played by different people over the years.
For example, I'm very sure that Peter Davison got his feeling hurt replacing Tom Baker, feeling Wesley Crusher kind of hate. I'd like to see his story, as he warmed to his own role over time perhaps, or poor Colin's health.
I'm sure each actor has gone through interesting things over the years...