Yeah, I think it was also because he finally showed weaknesses. Before it was like nothing could really get to him, nothing could get past his 900-year-old Time Lord armor, even in the most desperate, horrible situations, and then bam! Waters of Mars where he shows hybris and then fear, real fear, the most ancient feeling since human race gained consciousness: fear of death. When I say tragedy, it's of course in reference to the original meaning of the term; that episode was something the Ancient Greeks would have applauded to, no doubt. It was absolutely beautiful, the way it was handled, and I'm forever grateful to Russell T. Davies for managing to put this masterpiece together. This is the stuff of legend, seriously; the British really are lucky to be able to watch such incredible television.