Re the human sacrifice issue (@Bigjohn and Hookus) -- good points both. I did mean demographic impact, and I certainly don't disagree that human sacrifice was central to the culture and fairly widespread. But I do think Cortez needed to head off any criticism of his expedition -- not only its morality but to justify the expense -- and that his reports should be read, not as a pack of lies, but as an "spin" effort to make him come off looking as good as possible. When one sees 100 skulls displayed, there is the question of whether those are the victims of the last week's worth of sacrifices or the last century's.
I do think Bigjohn may have a good point about the moral impact of a human sacrifice regime. The Aztec empire, as I understand it, was tribute-based (rather than tax-based like the Roman Empire, for instance, became) and an important part of that tribute was sacrifice victims. It does seem like it would be hard to sustain such an empire long-term. What motivation would conquered peoples have to become loyal to their former conquerors (as provincials in the Roman Empire did)? Cortez succeeded in part because he found numerous native allies who figured anything was better than continuing to send their children to the Aztecs. Even so, I suspect that this may have been a case of a traditional practice spinning out of control, the Aztecs demanding more and more victims, and thus causing destabilization.
But my field is medieval Europe, so I don't really know with much confidence on this issue. Good food for thought.