1. The original title sequence from 1963 kicked off the episode.
2. Totter's Lane, where the First Doctor parked his TARDIS while his granddaughter Susan Foreman was at school, got a lovely wink at the opening of the episode.
3. And here's Coal Hill School itself, where companions Ian Chesterton and Barbara Wright taught. Look closely...
4. And we got a lovely little homage to TARDISes past via "the round things."
5. Although did you catch them again here, with Tom Baker himself?
6. Speaking of TARDIS redecoration...
7. A machine that goes "ding" was back.
8. These Queen Elizabeth gags were finally explained! Doctor, you naughty boy.
No wonder she wanted your head in The Shakespeare Code.
9. The Doctor's phone number has stayed the same since Season 4's "Stolen Earth," at least.
10. Ten's last words broke our hearts a little bit all over again.
11. The Eleventh Doctor actually carves the date and time of the first-ever episode of Doctor Who into the tower's wall.
12. And Clara drives past a clock displaying the time the first episode aired once more.
13. Clara's whiteboard at the start of the episode already had "no more" written on it.
14. The fez was, of course, quite present.
15. And these glasses sure do look familiar.
16. Kate's not the only Lethbridge(-Stewart) who's had to deal with multiple Doctors.
17. River's shoes are apparently a weapon of mass destruction worth keeping in the Black Vaults.
Not that this is actually surprising, come to think of it. Via.
18. BUT! So is Amy's pinwheel.
19. And then there's a brief appearance of Torchwood's magnetic clamps, as used in Doomsday.
20. Many of the Doctor's faces and Companions are visible on Kate's board.
21. Captain Jack, too, got a nod.
22. Arthur, the horse from Girl in the Fireplace, may have made an appearance.
23. The polarity got reversed.
24. The War Doctor's regeneration was a direct echo of Hartnell's final lines.
25. Bummed you didn't get to see Hurt regenerate into Eccleston? Fans believe if you look closely at Hurt's right eye in this scene, it actually does morph into Eccleston's.
When the flames start to cover his eye, watch the eyebrow. Something definitely shifts.
26. The time line of this episode ties in wonderfully to season one's whole "Bad Wolf" arc.
27. Tom Baker's surprise cameo as "The Curator" basically broke the fandom.
28. As if we weren't already done in by Capaldi's eyes alone.
29. Speaking of, Capaldi is starting to make a wonderful amount of sense...
30. And if you watched The Five(ish) Doctors Reboot (which you should), you may recognize the shrouded Zygons as none other than a few of the other classic Doctors.
Maybe. It's not been confirmed, but it's fun to think that may be a vaguely disgruntled Peter Davidson under there. Via.