The truly grim movies at Marvel are the X-Men films. In the Marvel Cinematic Universe, only Iron Man and Captain America have some ominous, grim shadings to the overall story (terrorism, espionage and government shadow cabinets creating paranoia) but the work of RDJ lightens the mood considerably. The Winter Soldier has been the most Nolan-like of the Marvel films to date.
The Hulk films were dark & twisted, in the classic tortured monster sense, but both were considered mixed bags by audiences and critics. After what we saw in Avengers, going forward successfully with the Hulk will be all about Banner & Hulk working together - 'How I learned to stop worrying and love the Hulk.' 😄
Conversely, when DC & Warners tried to get fun and goofy with Green Lantern, it backfired (that, plus a terribly fractured storyline.) A better balanced should be struck. Unfortunately, Zack Snyder won't be the one to lighten things up, going forward with the Justice League characters.