Also, some of the people I've talked to about it thought there was a possibility that Khan wouldn't necessarily turn "good" but wouldn't cross Kirk. These aren't dumb people, but it seems that those of us who have a more obsessive eye to our entertainment that they miss all the little "tells" in the story that would indicate Khan a true villain.
How is that a bad thing? Even I
spent numerous moments during my first viewing trying to decide if Harrison was going to help Kirk, hinder Kirk or pretend to help him.
I wouldn't have picked one of the villains in ST VI either, thanks to "Cinefantastique" magazine putting a big spoiler in a caption to a photo before I got to a sneak preview of the movie.