“…Literally the only semblance of Ngo’s draft to be found in the final film is the fact that there’s a drunken superhero named Hancock and a family (man, wife, son). That’s it. The characters act and talk entirely different, and just about every plot point has been altered significantly. What was once a harsh, bitter, intelligent, and thought-provoking character study of a washed-up, borderline-psychotic superhero became something else entirely. It became two halves of two very different films. And while that first half of Hancock proves that not all of the changes made by the studio-hired writers were necessarily bad, the second shows the obvious effects of having way too many cooks in the kitchen. Particularly when those cooks are fucking idiots.”

Hancock [SPOILERS]

And here’s a similar article which goes into more detail about the original screenplay.