Re: What exactly is Regeneration like to the Doctor?
I found it interesting in "Let's Kill Hitler" and most recently in "The Angels Take Manhattan" that they express concern about "using up" regeneration energy. River uses all of her remaining regenerations to save the Doctor's life, and the Doctor uses some of his regeneration energy to heal River's wrist. River freaks out about this. What would happen if he had used too much? Would he not be able to regenerate? Would he regenerate wrong?
That's a good question because in "Rise Of The Cybermen" The Doctor uses regeneration energy to power the TARDIS back up. He says it took ten years off his life. So maybe that's the only consequence, a shorter life span.
"India must be the future because we all know there are no neck ties in the future and everyone can do math"