-the Doctor's breakdown at the end: yes, I realize it had to happen somehow, in order to push Brooke and for RTD's intended end-show goals, so I can forgive its necessity. How it was executed though, seemed really out of character, artificial, and overwrought for Ten. Yes the Doctor is arrogant, yes he's powerful, but the way he quickly and rapidly descends into believing his own mythology seemed psychotic without much explanation as to why or how he got to the breaking point (esp. for a character whose usual MO is to always come up with the right plan at the last moment).
That is ultimately what destroyed this episode for me. Since I couldn't buy into the sudden personality change of the Doctor the whole episode rung hollow for me.
For me, it wasn't enough to completely dislike the episode, but I can definitely see where you're coming from. It's pretty jarring, to say the least.
I thought it made perfect sense, actually. The Doctor's been emotionally traumatized ever since "Journey's End" in the first place. He's been wandering time alone for goodness knows how long. He's been told that he's doomed to die, and so he's scared out of his mind anyway. And then he meets Captain Brooke, one of the most amazing people in history.
He's been looking for some sort of intellectual excuse, some sort of justification, for breaking the laws of time and intervening from the beginning of the episode. It was clearly tearing him apart to leave them. And what happens? He's away, he can hear them dying... and he has no companion around to hold his arrogance in check. Donna's not there anymore to tell him when he's going too far. And he realizes -- holy shit. I really am the only Time Lord left. I can do what I want.
Because, on some level, he knows he's nearing his end. And so he snaps. He goes too far -- because unlike with the Racnos, Donna's not there to stop him.