Disappointing. Is it really that hard to come up with new characters? Just taking a bunch of characters from the best star trek film and rehashing the plot isn't good enough. Lazy at best.
And I agree, Khan is from 1996, and is still frozen, so has no reason to be a vengeful mass murderer. Abrams has not added anything new to star trek, only taken various bits from previous films/episodes and rehashed the story. Why not a new race from another part of the galaxy, or even another galaxy? :/
They explain his motives and you even feel like he might even be a bit of a good guy. He has every reason to do what he does in the movie.
Until you remember who he is, Spock Prime ends up warning Spock about him which is when you remember how much of a cunt he is. It's not really a rehash either, the plot is very different from either TOS or TWOK. I'd say the ending is more of a homage.