I think the idea behind Doctor-Donna was to retcon/disprove the plot device mentioned in the DrWho movie that fans refused to accept - the Doctor being half human. By showing that a human/timelord hybrid could not survive, it effectively over-rules the existing concept and proves that it couldn't be true, or must have been a mistaken assesment by the Master (more fans would now be able to say it now isn't possible, and reject that one-off quote by the Master).

I think the real thing that fans should really be debating about on the final episode, is if he has now used up one of his 12 regenerations.
He said himself that the 'regeneration energy repaired himself, but didn't want to change his form because he liked it, and syphoned that part of the energy to the hand'. His body *generated* the regeneration energy AND it repaired him, like any previous regeneration. Just because he still looks the same, it doesn't mean he didn't regenerate in the visual sense. The Timelords have (apparently) a standard 12 regenerations, and each one could be seen as being like puberty, you can change a lot or a little (or in the doctors case this time, not at all). It is the repairing of a dying Timelord's body that they can only do 12 times, which is what a regeneration is for, not just to the changing of appearance - after all Romana did it a few times in one episode like changing clothes, and it wasn't regenerating. Changing form is obviously something they can do by choice, and not something that has to happen with a regeneration. So in the final episode, he had the regeneration energy repair his body that can only happen 12 times to a Timelord, which effectively means the 10th Doctor is also the 11th Doctor.
I think this is something that the writers will not acknowledge, because they will see regeneration as a visual thing, not an 'energy' based concept, and is gonna bug me the next time they have to change actors and call the next one the 11th Doctor.