I very much like the fact that they didn't just forget Tennant's secondary regeneration, and are playing around with the idea of the Doctor having side effects from entering his thirteenth regeneration. Evidently it's bound to be different and all of these little oddities that aren't usually part of the Doctor's regeneration sequences seem to scream that there's going to be something off with Capaldi's Doctor, at least at the start.
First thing to address was the power and strength of the Doctor's regenerations, over time they've gotten stronger and stronger (More than likely because they're using more and more of the Time Lord energies that he's got stored inside him with each additional regeneration. So the fact that the Town of Christmas was almost destroyed in the process of his regeneration's not really surprising and seeing as that was the major change for him (the kick to start it if you will) then the swiftness of the change from Smith to Capaldi shouldn't come as a surprise.
The two things that this episode shocked me with were tiny, little things that most people wouldn't notice or think that they're just in there for the fun of it. The first of these little sequences is when the Doctor tells Barnible-who's-not-really-Barnible that he's got a plan up his sleeve, but after Barnible leaves he reveals to Clara that he in fact doesn't have a plan and people just like it when he says that he does.
The problem with that scene is that the Time Lords are supposedly projecting a massive truth field onto Trensalor which prevents him from lying AT ALL. So how on Earth was the Doctor able to lie (While sitting right next to the crack in the wall where all of the transmissions are coming from) without breaking a sweat? I mean even after three hundred years of constant exposure we still see that the Doctor's still unable to lie or hide anything about himself (telling Barnible that he might leave tomorrow, or the day after that, or the day after that) which proves that he can't have just become immune to it all.
The second sequence is of course the moment that Capaldi says "I don't like the colour" referring of course to his new kidneys and then going on to say "You don't happen to know how to fly this thing?". The Doctor never refers to the TARDIS as just a "thing" and in all of his previous regenerations, he's never once lost the knowledge of how to maneuver and pilot it. He's also always had to check his body to see what things looked like, yet he somehow knows what colour his kidneys are? Is this some sort of special ability that he's gained from having an abnormal amount of regenerations? And is it somehow tied in to him not being able to remember how to fly the TARDIS?
Whatever the case may be I'm desperately hoping that Moffat addresses these issues with Capaldi's first episode, and I'm also very much looking forward to seeing just what suit he chooses. The Doctor's attire is what makes him such an amazing character and the fact that bow-ties and fez's' are no longer cool makes me kinda sad, still it'll be interesting to see what Capaldi decks himself out with next series.