I agree that it takes time for an actor to "become" the Doctor, and it also takes time for the writing staff to start figuring out how to write to the actor's strengths and to "get" the conception of the character.
The best Doctors, I think, tend to bring much of their own approach to the character, while the staff also has a good sense of where the character is going. I think the best example of how this DOESN'T work is with the Sixth Doctor where the entire schtick was just him being a jerk. They had similar problems with Adric, where Matthew Waterhouse was probably too young to really bring his own sense of the character to the show, and the writers had no idea what to do with him.
I think that's actually been one of the greatest strengths of the new run of the show -- the writers and actors all have a much clearer sense of who the characters are and why they do what they do. Chris Ecclestone's Doctor was fantastic in this regard, although short-lived. He played just enough mania mixed with melancholy to really bring across that he was dealing with the guilt of being the last of his kind and having been the one who killed his own race. But the writers had a really clear vision of this too, and you can see that with the threads that unite Ecclestone's performance in "The End of the World," and "Dalek."
Tennant, I thought, took a bit to find his own footing, but once he did, he brought a far more emotionally unrestrained version of The Doctor to the screen. Whereas Ecclestone had seemed to be holding back his own rage and pain and sorrow, Tennant took the character forward and allowed himself to be more...there, emotionally. I got the sense that the Doctor had gotten to a point where he could just feel more instead of trying to forget. He'd learned to live with his loss, mostly, until the very end where his losses mounted to a point where he just couldn't deal.
Smith's run, by contrast, has been much abridged with only 13 episodes in the first season and 14 this season (and 14 next season). I think Season 5 was far more action-driven and gave Smith a lot less time to really explore who the 11th Doctor is, but we're starting to get a better sense of it now. I hope he has enough time to really own the role and that the writing staff stays solid enough that they "get" who he is and what he's doing. Smith's performance is far less weepy and joyful than Tennant's, but I think he's continued much of the mania of the Doctor forward, as well as some of the childlike joy that was there from the 9th forward. (And possibly the 8th...we hardly knew ye...)
Anyway, I'm looking forward to seeing more of what Smith & Co. have to offer. I'm enjoying the 6th season so far, having just finished Day of the Moon (which was pretty...wow...where is THIS stuff going?!).