There are number of things that are done right. The cinematography and color palette are perfect for the film. Tilda Swinton is excellent as the angel Gabriel, as is Rachel Weisz (double cast as the mystery-hunting cop and her dead sister). Yet all the credibility the film manages to gain, it turns around and squanders on unneeded embellishments and alterations. For example, the character of Constantine is notoriously self-serving and self-destructive. The man has virtually no power when compared to the occult foes and demons he opposes; he gets by on luck and sheer bravado. He makes dangerous and stupid choices because he is almost certain he'll be able to cheat death.
Despite all this, however, director Francis Lawrence apparently felt the need to make his lead character sympathetic. As a man who can meditate himself into hell, apparently Constantine isn't macho enough -- so he's fitted with the Holy Shotgun of Antioch to wreak holy tommy-gun vengeance on the demon half-breeds he needs to deport en masse to hell.
Worst of all, Hollywood takes the man who manages to trick the Devil into curing his lung cancer and uses him to let the kids know that true tough guys don't need to smoke in an uncalled-for after-school-special-esque ending. I hear that in a very special sequel, Constantine will illustrate the danger of cheating on your math test.