According to Memory Alpha, Augments, as compared to normal humans:
* were five times stronger
* were more agile
* were twice as intelligent
* had greater resistant to sickness
* had greater sensoral acuity
* had heart muscles with twice the strength
* had lung efficiency that was 50% better
* had twice the lifespan
All these could be accomplished through advanced genetics. We are already changing the nature of animals and plants through genetics. Given time, we might be able to do the same with humans.
Khan has suddenly, in this new film, become Jesus. He has this remarkable dunamis that, if tapped, could revive the dead. We have moved past genetic re-sequencing and have moved into the supernatural.
Eh, I kinda like that, though. It may not necessarily be "realistic" (though I don't really demand realism from Star Trek) but it definitely gets across the message that Khan is an enhanced being.
All we really saw before was that the Augments were stronger and more resilient. They didn't seem *all* that superior. They were basically human Klingons.
This guy is Nietzsche's superman, though, and that makes him terrifying.
I think what Abrams has done for the franchise is write Trek *as it would be written now*, rather than try to deal with dated concepts. And I feel it works.