No-one has been able to clarify why the fuck everyone thinks Khan was so bloody brilliant. He was a bad captain who made all the wrong decisions, had no idea when to shut up and was too self-absorbed and plain stupid to keep his intentions a secret until nobody could stop him anymore.
Khan was a stand out villan because he had depth and could be understood. He was after Kirk for a very personal reason and that was emotionally realised on screen, unlike Nero. You say he made all the wrong decisions and was a bad captain. He couldn't have been that bad if he took over the Reliant and stranded her crew. But his errors highlight a very important part of the character. He egomanical approach is what was his undoing. He let vengeance consume him but refused to realise it because he thought he was smarter than everybody else. I think the real difference is in portrayl. Montablbans role was well written and well acted and had a great deal of screen time. Nero, on the other hand, get a lousy short flashback for us to understand his anger, and the odd scene with Pike. You don't feel anything for him because by the time he dies you hardly got to know him. Whilst Bana does his best with the role, the character simply has no development whatsoever. Even Shinzon, dare I say it, had more time for the exploration of his character than poor Nero did.