TWoK established that both characters had a backstory. If an audience member wanted to see that backstory, he or she could buy a VHS copy of Space Seed. (In conjunction with the movie, Paramount had released a VHS copy of the episode.) [
http://en.memory-alpha.org/wiki/Star_Trek:_The_Original_Series_(VHS)]
Classic Trek Khan was a ruler of people. As a successful ruler - who never experienced internal dissension in his territory - , he had to have the ability to negotiate for what he wanted.
I think using Khan was a mistake. Khan isn't the Joker. What is the Joker? He is a master criminal who is a nemesis to Batman. Nolan could craft a Joker whose backstory we never learn and make a film that works, regardless of the plot holes. Both characters were born into, lived in, and worked in a world of the same technologically level.
What is Khan? He is the product of the Eugenics Movement in 20th Century Earth, who with hundreds of others managed to conquer both Asia and the Middle East. Their inability to be satisfied with their conquest lead to the Eugenics War, where they were out competing each other to be the top dog. How does one work that into a film, without boring the hell out of a contemporary audience?
** First, there has to be an explanation for what Eugenics is. I doubt very many nowadays would know what the Eugenics Movement was.
** Then, there has to be a reason why a person in the 23rd century should be afraid of a man and his small band of followers from the 20th century. They are outmatched technologically. (The only reason that Khan was able to commandeer the
Enterprise is because Kirk, acting like a fool, gave Khan access to the engineering specs on the starship. Even Spock questioned Kirk's judgement on that one.) It was believable in the 1960s that a man and his followers could escape with nary a record trace. It is less so today, where it seems every action of ours is being tracked and recorded. Even our choice of entertainment is leaving a record of us behind for someone to find. We are losing anonymity for the sake of advancing our technology. Realistically, if a 23rd century ship came upon the SS
Botany Bay, the crew would identify the occupants and quarantine those people asap.
** Thirdly, the 23rd century would have means to counter the superiority of Khan. The purpose of Eugenics was to create people who were superior. Well, technology augmented to our living frames can do the job just as well. So, there would be a fight between naturally augmented humans and cybernetic augmented humans. Would that excite the audience to see this fight? And would this still be Star Trek?
I feel the threat of Khan reflected the fears of people who grew up in the WW II era where people knew about the Eugenics Movement of NAZI Germany and the fear of Asian imperialism coming into America were commonplace.
Iron Man 3 faced a similar challenge with the Mandarin. The Mandarin reflected the fears of 1960s America - fears that didn't make sense to a contemporary audience. So, the filmmakers took a risky step with the character, and upended the whole Mandarin character arc.