The look of Hell was based on old footage of nuclear tests, specifically the sudden shockwave immediately after the blast that disintegrated anything in its path, hence the crumbling landscape.
The character of John Constantine was originally created by Alan Moore, during his run on DC Comics' "Swamp Thing". However, following his negative experience with From Hell (2001) and The League of Extraordinary Gentlemen (2003), Moore decided to reject all money and credit from Hollywood on any adaptations of his work. Thus, he gave all the money he would've gotten to the artist who drew the character with him, and rejected his own "created by" credit from the film.
(at around 40 mins) According to the director's commentary on the DVD, the rag that Constantine lights on fire to defeat the flying demons is a piece of Moses' shroud, given to him by Beeman.
Domino Harvey: (at around 30 mins) the model and bounty hunter, basis for the film Domino (2005), makes a cameo at Papa Midnite's Bar. She is seated at a table as an angel, with a demon nibbling her ear.
Michelle Monaghan: (at around 1h 28 mins) She can be seen briefly in the hospital scene as the one who exclaims, "Holy Water?" Her character, Ellie, originally had a larger role, but her scenes can be found in the deleted scenes section of the DVD.