Some choice bits from IMDb's trivia page:
*The Baseball Furies were created due to Walter Hill's love of baseball and the Glam Rock band Kiss.
*The film trucks were "protected" by a real gang called The Mongrels for $500 a day.
*James Remar earned his role as Ajax after becoming so involved in the audition reading of the park bench scene, that he lifted the massive table around which the director and producers were seated.
*The crew once got urinated upon from a tower block due to the noise they were creating in the night.
*When the fictitious Turnbull ACs visited a hamburger joint during filming, people fled in fear of being attacked.
*The Homicides were a real Coney Island gang, and they didn't approve of fictional gangs wearing colors on their turf. The wardrobe department made sure nobody walked off location wearing The Warriors colors. The actors were safe during the cemetery scene in Brooklyn because of a fence surrounding it.
A gang name in the script that probably didn't get a mention in the movie: The Xylophones.