1. Gibson's particular prose style was revolutionary in genre fiction when it was introduced. Remember, the default in genre was 3rd person omniscient and American SF was not receptive to the New Wave that happened in Brit SF a decade earlier. It was a legitimate shock to average readers, introducing them to a lot of literary devices that were foreign to readers at the time, and that reputation stuck a bit.
2. Neuromancer actually marks a split in mainstream SF, especially in America, where "hard" becomes a more contentious label than before. For around 20 years, mainstream SF was predominantly "hard" science fiction in the tradition of Asimov and (to a lesser degree) Heinlein; it wasn't until the Cyberpunks started getting published that the "space ship story with elaborate physics" was challenged in genre publishing, after the New Wave failed to change American SF publishing in the late 60s and 70s.
3. Neuromancer was one of the first novel length products of the Cyberpunks. We don't really remember it now, but the Cyberpunks were a literary movement, with a manifesto and social network, a lot of editors and publishers, and an explicit goal of changing what was published in SF. In that context, the "difficulty" of Neuromancer was exaggerated in order to add literary cache to the movement as a whole.
4. So much of what was revolutionary in Neuromancer is now commonplace. The way Gibson builds worlds (with throwaway details that give a sense of lived in settings), the way he obscures parts of his narrative or leaves details out, the sociological layer added to SF and the removal of the "technobabble" that had come to dominate the genre are all common place now, part of the regular tool kit of SF. We're two, maybe three revolutions on in SF (Cyberpunk, New Space Opera, MFA science fiction), and Neuromancer has been a must read for every one of those waves, so it's DNA is basically like Genghis Khan's is in the real world: all over the place.

If you want to see what he's been up to lately, I think the Peripheral is his finest work since Count Zero, and melds his newer style of work with his more SFnal early stuff. I'm not a fan of the second book, but Peripheral is the purest distillation of what he adds to the genre as a whole, even decades later.