I thought it was never really explained that Khan was previously a tyrannical dictator from Earth's past. Maybe I missed it, I've only seen it once, but I got the impression that one would only know that if they remembered it from the show.
Also, he says that he never wanted to be woken or something like that and I think in the show they had originally set out to find a new planet to settle on. So something like that would have been good.
And not really dialogue, but I would have loved it if Spock's neck grab had instantly sent Khan to the ground. Not too exciting I know, but it would have been better for the character and the good kind of anti-climax.