I honestly have no idea where to even begin with this episode. The opening scene with Clara and all of the incarnations of the Doctor gave me a raging nerd boner that was compounded by the unexpected appearance of Professor River Song. To use internet speak, this episode gave me a lot of "feels". Then it destroyed those feels like Emperor Palpatine destroyed the Jedi Order - Quickly, brutally, and without remorse.
Predictably, they managed to avoid revealing the Doctor's name, which should surprise no one at all. Personally, I think his real name is Leroy Jenkins or something. Then Clara had to go and be a Big Damn Hero and catapulted herself into the highest echelon of companion, somewhere in proximity to Rose Tyler and Donna Noble by seemingly sacrificing herself to save the Doctor. I absolutely loved, loved, LOVED her interacting with the First Doctor and convincing him to steal the right TARDIS. Also, holy shit, we got to see what the TARDIS looked like before it became a police box! Never thought that would open.
Easily - easily - the biggest shocker of the episode was this:
My two theories: He's The Valeyard, mostly because I've been wanting to see that character in New Who for a while. What can I say? I just like the idea of an evil version of the Doctor. The problem is that this doesn't act like he's the embodiment of the Doctor's darkside. He came off more as being sorrowful, full of regret and remorse and whatnot, which leads to my second theory: He's just all of the Doctor's regret, rage, sadness, guilt, remorse, etc. that the Doctor experienced fighting in the Time War and trapping Gallifrey (and his entire race) behind the Time Lock.
I've had a belief for a few years now that the Eighth Doctor survived the Time War, but regenerated because he couldn't mentally handle the horrors he witnessed, the things he did, and the way it ended. Maybe he wanted to try and start fresh with a new body and personality. Maybe it was more dire and he started having a complete mental breakdown and some part of him decided that his Eighth Incarnation didn't deserve to suffer any longer and so he regenerated as an act of mercy.
Of course, we won't find out until November 23rd when the 50th Anniversary special airs.