The idea of Regeneration is probably one of the most brilliant plot devices ever invented in the history of broadcast television. His appearence is altered, maybe severely in the case of the Doctor, but the central hero is still the same character, the same man, erm, TimeLord, the same being at his core.
This concept was understood in the transition from 9th to 10th Doctor. Eccelston's Doctor goes out of his way to explain to Rose that yes, he's dying, but he has a way around it. Yes, he's going to change and she won't see him again...with that face. And, reinforcing something I've always personally believed to be an unspoken truth about the Doctor -- he sucks at regenerating, so he can't be sure what he'll get when he does it. Rose doesn't get it, but maybe the new viewers to the revived show do, and off the Doctor goes, like a gasoline fire, burning away the old dying body to make way for the new...
"Hello... New teeth, that's wierd."
The Doctor's first words to Rose Tyler after he regenerated for the 9th time to save his life after absorbing the energy of the time vortex. And why wouldn't it be wierd. It's the same guy within the context of the show. Again, Rose -- with whom the audience is meant to identify -- is the one who needs to be convinced this is the same man she fell in love with the previous season. Her mother and Mickey get it, Harriet Jones Prime Minister gets it, and eventually, Rose comes around as well:
"I'm still the Doctor then?" he asks after she throws him a sword in his Christmas Day duel with the Sycorax leader.
"No arguements from me!" she replies. And there ya go. It's the Doctor, regenerated and ready for more.
So what happened between then and the next regeneration?
Don't get me wrong, I loved both parts of the End of Time. I though the master went out EXACTLY like he should, I'm totally on board with Rassilong back as President, I think the loose ends of the Time War were sewn up nicely -- leaving the possibility that a few clever time lords may have bailed fro mGallifrey while they could -- David Tennant's performance as the Doctor kicked ass as usual... but then things take a crap at the regeneration.
Without ripping into both episodes with needless nitpicking, I'll simply say I was VERY dissatisfied with the tantrums the Doctor threw, not because he had them, but because the accompnying lines were lame and out of character. Being worried about regenerating is nothing new to the character. Being a narcissitic ass clown is also nothing new to the Doctor when he's real pleased with himself, or wearing his 6th rengeneration. It's in character and often funny. But not this time. This time it seemed out of context not only of the episode but of the whole history of the show.
"I don't want to go."
You don't want to go? What the hell are you talking about? You're afraid to go through with it, okay. You're afraid you might not make it, understandable. You hope you don't forget everyone you cared about, it could happen. But you don't want to go? You mean you the Doctor or you David Tennant the actor. It was a bizzare, out of character way to handle things and it didn't do Matt Smith, the next Doctor, or the next season of the show, any favors. An actor's departure was emphasized which is in total opposition to what the plot device was invented for. Yeah, one of the best actors to portray him has moved on, but The Doctor survived his regeneration and so should the show.
Oh well, still a great episode all told. And seeing Rose last before he regenerated was a massive nod to that character's importance in the revived series. River Song can get bent, Rose and the Doctor all the way!