What kills Hancock is the direction. Berg's snatch-and-grab semi-documentary style worked gangbusters for Friday Night Lights, but for the super-heroics of Hancock, you just get the impression that they're trying to hide some flaw in the effects from you, especially in the inevitable fisticuffs that these things seem to depend on (although the cleverer ones find ways around it).
Will Smith plays John Hancock, who can bend steel in his bare hands, but would rather bend his elbow. He's alcoholic, down and out and super-depressed. Most super-heroes are high and flying, but Hancock takes it the wrong way, so rather than looking up in the sky for him, chances are the best place to find him is the gutter. He has powers and abilities far beyond mortal men, but no idea who he is, how he got that way, and there's no messianic father-figure to tell him the back-story. And yeah, he'll do all that super-heroic stuff, but he's too stoned out of his mind to do anything carefully—he's like a bull in a china shop, and every time he does some good, he does a lot more harm in property damage.
After saving the life of marketing maven Ray Embrey (Jason Bateman), Hancock is taken under wing, sobered up, read the "with great power comes great responsibility speech," and given a better public image (rather than a secret identity), a leather "sooper-suit," and is supervised in his derring-do so that he first does no harm.
Hancock isn't very good, but it has an interesting take on things "Superman-ish," and when you see Man of Steel, the latest "official" Superman movie, you'll see some of the ideas brought back to the source—the colliding bodies, the city-wide collateral damage, and the hard edge of sci-fi aspect to things that most super-hero movies tend to miss for the musculature and myth-making. Also, the script has a literal vulnerability for the super-dude that's far more ingenious than sticks and kryptonite-stones. You wanna reduce Superman's effectiveness? Attack him through his loved ones. Hancock takes that one step further by making his loved ones his true weakness, making him vulnerable, and shortening his life, taking the "apart-ness" of the super-hero, bringing it from the sub-text to the foreground, and lending the whole thing a nice melancholy touch that's missing from the genre as a whole. It also brings Will Smith nicely down to earth, as well.