Gibson borrows his writing style from the hard-boiled detective novels of the pulp era, stories written by guys like Raymond Chandler and Dashiell Hammett. These detective novels were written in a conversational and street-level style and were filled with slang, nonstandard dialects, and, best of all, cussing. Here's an example of what we're talking about:
"I wanna buzz," he said to the blue artifice of the sky. "I truly do wanna get high, you know? Trick pancreas, plugs in my liver; little bags of shit melting, fuck it all. I wanna buzz." (10.119)
Yeah, if you're inner prude cringed a bit there, that was precisely the point. And if you're inner grammar policeman cracked the whip, that's quite all right. Case uses the more street "wanna" instead of "want to." He speaks in short, terse sentences, and his language isn't exactly the polite conversation found at the family dinner table. It's a far cry from the Standard English of your typical cardigan-wearing professor.
Although the above example doesn't have any slang in it, there are plenty of examples of slang littered throughout the novel, such as "flatling," "console cowboy," and almost all of Maelcum's dialogue. In fact, slang is so prevalent throughout the novel that we were able to make an entire quiz for our "Quizzes" section dedicated to nothing but slang. Best of all, Gibson was such an original writer that he invented much of his future slang himself. Sure, he borrowed from various places, but it's the way he put it all together that makes Neuromancer's style such a bona fide original.