I blame two factors:
1) There was a large block of exposition before the plot kicked into high gear, the kind that both completely derails the pace of the movie, and that makes you go "wait, that ... doesn't really make sense."
1a) There was no flow between the fight scene with the cement truck and the next bit of heroism - it was like, "here's one thing. Ok, now here's another."
2) The relationship between the characters was in a weird meta-state that basically disallowed playing up any kind of attraction, pretty much anywhere. In fact, the weird situation introduced in the middle of the movie somehow silently resolved itself by the movie's end, without nary as much as a whisper. The awkwardness just slunk away and said "ok, enough of that plot point, we can pretend that stuff never happened."
Also, there's a good chunk of time unaccounted for in Hancock's backstory. Utterly ignored, unexplained, not even missed by any of the characters. For all the buckets of exposition that were poured on us, they missed some pretty big spots.
So, um, yeah. "Show, don't tell," is what I'd tell the director of this movie. There are some funny moments in it, and some "what the-?" moments...
Oh, one more (fourth of two?) factor: they failed to set up the main villain as anything but a punk with a gun and some kind of... er... planning ability? ...and almost all his setup was done as a news report playing in the background of some other scene.
It's utterly impossible that, at the time he plots an attack on Hancock, that he has any way of knowing that Hancock will be vulnerable... as far as anyone knows at the time he's making his plan, it's the stupidest plan that ever stupided a stupid. If it can be called a plan, really. It's more like Ray's plan for dealing with the librarian: "Get 'im!"
So, yeah. The reason this movie failed was because it was dumb, and it relied on audiences not noticing.
I am trying to decide whether to give Charlize Theron credit for having played her part well enough that it was possible to kind of predict the shape of things to come, from an early point in the movie... or whether I should ding her for overplaying her hand a little. Enh. Benefit of the doubt, there.