I'm excited to see the re-introduction of River Song. I just watched the 2 parter Silence in the Library. I think Matt Smith's introduction to her can make or break his Doctor (And Moff as show runner). He'll have to recognize her, but, keep it to himself. RTD made a big deal of her and who she becomes to the Doctor. Confusing thing is, though, she told David Tennat, she's never seen him so young? Matt Smith is literally younger than David Tennant and in the first episode at least, played the character as younger and more carefree. Certainly considering the DT episodes after Silence in the Library, the character "aged" through drama and loss, but, that seems to have been erased with the regeneration
RTD wasn't piulling the strings on that two parter... it was a Moffat story. RTD has publicly said (even in the DWC following the story), that you don't touch Stephen's scripts because they are "that good".
You need to look at The Doctor the same way that Moffat does. The reason he cast Smith was because despite his physical youth, he felt he could convey the idea of the Doctor being the wise old man of the universe.
It's not about the words or the actions it's about the soul. River (for whatever reason), connects with the Doctor on that level. If you think back to the second episode and what Amy said to the Doctor...
"Very old and very kind and the very very last. Sound a bit familiar?"
The Doctor often plays the manic jester, and yes his new physicality may contribute more to that... he also often skirts with the darker side of his nature when he switches to immense brooding anger... but at his core he is still The Doctor and he is still the wise old man of the universe.
Some people have bought the RTD "Even if I regenerate I still die, some new guy goes sauntering off" line a bit literally.
The Doctor's traits may or may not change with each regeneration... but at his hearts he is the very same person within.