Chrome! At least that was the official answer in the original cyberpunk game.
OK more seriously, the defining feature at least for me is an ecology of subcultures with very different opinions attitude and lifestyles. The future is everything that ever was, just more so.
Even more verbose version: Everybody is into some kind of overdrawn style. So you might be chased by a goon who is into a 1980 style, so he wears an ill-fitting 1980s suite, has an old fashioned pistol and drives what looks like a 1980 police car, only the suit is bulletproof, the pistol fires micro-missiles (but has a little speaker for the authentic bang-sound) and the car can fly. Around the next corner there is a group of punks, they look like 1970s-London but the wear little devices on their ears that project old sex pistols footage onto their mohawks. You run into a hotel that is roman republic themed. Guests are given a complimentary toga. On a screen Julius Cesar gives a speech. Of course it's not a recording of the real cesar, but the studio that produced it rendered it as a black and white silent film with old movie artifacts to make it look authentic (If it's not 3d it must be ancient, right?)
I think a story game needs a story, don't think open world would fly unless you made a huuuuge effort.
Recommended Reading / Viewing
- Blade Runner (Obviously)
- Strange Days
- Neuromancer (William Gibson)
- Snowcrash (Neal Stephenson)