Warning: Nerdy Rant
Here's my problem with this: From watching the earlier seasons (first series, before doctor 9) there's a significant amount of time the doctor spends in each body. For 400 years he was doctor 1, and doctor 4 lasted for 7 seasons. They established early on that a timelord can only regenerate 12 times (making 13 doctors total) but knowing the popularity of this series, they'll find a way around that.
The issue I have here is that we aren't spending enough time with these new doctors. The original series had story arcs that lasted 5 to 6 episodes each, and each doctor had several seasons of this, doctor 4 had probably 40 story arcs to his name. While David Tennant was in about 40 episodes as doctor 10, each episode was only about 45 minutes as opposed to 100. We're spending less time with each of the new doctors. Eccelston was only on for one season (12 episodes) Tennant ran with it for 3 seasons and 4 specials, which was a significant amount of time, but would be on par with that of one of the first three doctors, not doctor 4. And now Smith might only do it for 2 seasons? We're running out of doctors here.
At this rate, in about 5 years, Doctor 13 will die, forcing the writers to come up with some random loophole to keep him alive, or start a new chain of doctors, and they'll run through those even faster. Doctor Who is a fun and amazing series, and each actor seems to want to leave the role as soon as possible! It's not cool, man.