The Doctor: Oh, Doctor you're so handsome. Yes I am, thank you.
The Sontaran Stratagem (1) - S4-E4
Doctor: Name?
Skarl: General Skarl of the tenth Sontaran battle fleet. Skarl the undefeated!
Doctor: Oh that's no good, what if you get defeated? Skarl the not-quite-so-undefeated-any-more-but-never-mind?
Donna: He saves planets, rescues civilizatons, defeats terrible creatures. And runs a lot. Seriously, there's an AMAZING amount of running involved!
Chosen answer: 1) When Stephen Moffat took over he ignored a lot of what had been developed before (there is not in-universe answer). 2) It would have killed Rose, so the Doctor absorbed the energy. His body regenerated before the energy could do a significant amount of damage that would prevent regeneration.