The Marvel Cinematic Universe hinges on us caring about Tony Stark. He's such a major component of the mythos that his stories need to be handled with the utmost care. 'Iron Man 2' didn't exactly do that, so they needed to bring in the big guns: Shane Black. You may know him from writing 'Lethal Weapon,' 'Kiss Kiss Bang Bang,' and 'Last Action Hero.' OK, so maybe 'Last Action Hero' doesn't evoke greatness, but those other movies are great! He directed this movie, putting his stamp on it while also making something that very much feels like an Iron Man movie.
This movie really pins Tony Stark down as a hero on par with someone like Captain America. Considering future movies would have them face off, that's a very important component. Who do you got? Captain or Iron Man?