Cyberpunk

Cyberpunk as a sub-genre label was a marketing label almost
from the beginning, and hence, almost completely meaningless.
However, I'd say you need the following features to really
have a cyberpunk game:

1. Dystopian future. No happy ending. Maybe you win a battle,
but the general crappiness of the world stays untouched.

2. Silly Cyberspace. You have to spend a fair amount of the
game in a virtual reality environment. Forget everything you
actually know about computers; Gibson has said he could never
have written Neuromancer if he hadn't been computer illiterate.
This is actually a very good thing, since you need to including
hacking, and this approach is more gameable.

3. Corporate Dominance. The world is largely controlled by huge
multinational corporations, most (not necessarily all) of which
are corrupt and irresponsible (but not bwa-ha-ha-ha evil).

Note that not all listed sources have all these features. However,
for an indisputable cyberpunk game, you really need all three.
This presentation provides a somewhat cliched view of cyberpunk,
but while writers should avoid cliches, designers of a
genre-centric game should embrace cliches. They make easy handles
to get the player into the world. Innovative interpretations of
the genre can wait until the basic cliched version has been 
worked over a bit, at least.

Part 1: Ideas (largely the work of Ray Dillinger)

Skatepunks with razor-edged-frisbee launchers.

MicroWire whips.

Cyber-enhanced attack shrews.

Musicians with diesel-powered assault bagpipes and
     weapons-grade amplifiers.

A clone created by your medical provider, who escaped
    missing his right hand (it was handcuffed to the bed)
    and now wants to harvest *you* for spare parts.

AI's plotting to take over the world - but what
    cyberpunk game *doesn't* have those?

Shark brains repurposed for torpedo guidance.

Performance-enhancing drugs that have a cumulative
     deterioration effect. "live now, pay later..."

Cloned sex toys - gorgeous nymphomaniacs.  The problem
     is people who get these and then get bored with
     them, or just can't have as much sex as they thought
     they wanted, and abandon them.  The ones who've been
     abandoned (or "set free") are still gorgeous
     nymphomaniacs, but shunned by normal society, rejected
     by their owners, bitter about it, and consequently most
     of them resent and hate normal people .... they're
     actually pretty dangerous.

Cloned soldiers.  They're eight feet tall, they
     regenerate like mad, they're ridiculously strong
     and ridiculously fast, they grow to maturity in
     six years, and they die automatically before they
     turn forty (which saves retirement benefits....).

Clone Liberation Front (as terrorist organization,
     weapons dealers, medical spare-parts network for
     clones, legal advocates in clone-related cases,
     etc...)

Nuclear Family: properly speaking, nihilists rather
      than terrorists.  But the distinction can be
      pretty thin...  HQ is on the bikini atoll.

Camera flies: genetically engineered houseflies
      that broadcast a low-powered television signal
      with audio.  Frequently used to breach security.

Gas Rifles: Police weapons which fire gas grenades.
      Choice of rounds ranges from incapacitating to
      permanently disfiguring to lethal, including
      doses of various otherwise-illegal street drugs
      previously seized as evidence.

Legion of Dynamic Discord:  Practical jokers gone
     very, very overboard.  Consider it funny to
     spike a city's water supply with a mixture of
     amphetamines and aphrodisiacs and fund the
     operation by investing beforehand in the stock
     of the local brothels.  Would also consider it
     funny to organize simultaneous conventions for
     opposed hate groups in the same hotel (managing
     to offer the best rates to both groups by first
     shorting the hotel's stock...).  Their activities
     are closely but clandestinely monitored by
     investment analysts and securities brokers, who
     plant operatives in the group for the investment
     tips. It is unknown whether the group has any
     *other* members.

Investment Analysts as a source of industrial
     espionage work for the characters.

Monocycles:  The speed-like-an-idiot vehicle of the
     2050's.  They have one wheel and use gyroscopes
     for stability, control, and power storage.  They
     mount a small turbine engine, have regenerative
     braking, and are incredibly maneuverable.

Streetsweeper autofire assault shotguns.  With the drum
     magazine, these things are awesome.  When launching
     minigrenades instead of buckshot, they're devastating.

Needlers: blistering rate of fire and tens of thousands
     of rounds of very tiny ammo.  Individual needles
     don't do much, but getting hit by a thousand of them
     in a couple of seconds will wipe most of the meat off
     your skeleton. A relatively light armor will make you
     immune to these.  They are short-range weapons (effective
     out to 30 feet or so) and often used by security personnel
     indoors because they don't penetrate walls.

Water Wars: Climate change forced the third  and fourth world
     wars to be fought over water rights.  Perhaps these wars
     are ongoing in the time of your game; perhaps they are
     part of the history.

HMD sunglasses:  What they do depends on the software;
     combat programs are available to map routes through
     freefire zones, retrieve satellite or camera-fly
     views in realtime to give you the power to "see through"
     most things, plus IR and UV vision, and interface
     with your gun to put the crosshairs in your vision
     exactly where it's pointing.  Or they can make the
     best non-surgical interface available to a
     cyberdeck.

     Or you can get a consumer OS and surf the web for free,
     while being programmed to shop at the stores of the
     companies who paid Redmond Software for your business.
     And incidentally programmed to not mind that.  And also
     programmed to buy the next upgrade from Redmond
     Software....

Radio-coded pacemakers:  Often an inescapable part of a
     lucrative employment contract; don't worry, your boss
     won't turn yours off unless he hears that you've been
     working for the competition.

Assassin vending machines:  Vending machines that dispense
     robot assassins (only found in FreeTrade Zones).  You
     have to give the system a photo, a name, and an address
     when you deposit your money.  The robot will "wake
     up" after you leave, not knowing who you are.  They
     have enough power to operate for six weeks.  Some of
     the better companies advertise a "clearance percentage"
     in the high seventies.  You can provide extra weapons,
     vehicles, or tools for your assassin 'bot if you're
     willing to spend the money.

FreeTrade Zones:  After several decades of wrangling about
     jurisdiction, controlled substances and items, and laws
     that disagreed about what transactions were illegal, the
     problem sort of solved itself, in the form of FreeTrade
     Zones -- areas where *NOTHING* is illegal.  FreeTrade
     zones grow up where governments are weak, overextended,
     or crippled by jurisdictional or diplomatic problems;
     some permanent FTZ boat communities now exist in
     international waters, and land-based FTZs exist in
     areas where the governments are nonexistent, powerless,
     or can be bribed to look the other way.  And BrightLand
     Station, parked in High Earth Orbit, is notorious as the
     FTZ of the ultra-wealthy.

if it's a near-future game, be it cyberpunk, post-apocalypse,
post-cyberpunkalypse, or other, you could have "drunk" and "pimp"
player classes. "Stoner" and "whore" too while you're at it.
Not everyone can be a "cybersamurai" or "hacker".'
[this one's me, RDH]

Janitor would be another possible class, or Corporate Deadwood
for a clock-punching middle manager.

I want (somehow, somewhen) to allow using duct tape to bind together
several kinds of weapons, so that you can wield it and shoot
simultaneously, just like at the end of Aliens 2. [-- the Sheep]

Cybernetic implants: "humanity loss" idea is common drawback,
but largely arbitrary and unrealistic. There are realistic
problems for cyberware:

For weapons, consider size limitation, difficulty of loading
your arm, inability to drop weapon if you want to surrender,
heat problems, etc.

[quoting another of my old posts] I've thought of cybernetics
as the sci-fi equivalent of fantasy "mutations" or "chaos
thingies", although not so random, of course. Many
could translate directly over and the value of many of the mutation
weapons is that they give *extra* attacks. So I'd hit with my mace and
*also* slash with my cyber claws, bite with my cybergoth pseudovampire
fangs, and lash with my cybertail. All at once. Instead of guns in my
arms, I'd leave those free for holding my pistols, while firing off
poison darts from the dartgun in my skull and, against tough opponents,
firing the low-level nuke "crotch rocket" installed in my groin. 

How about some "reactive weapons". A subdermal network of electroshock
weapons would active when your skin was penetrated, giving that rat
that just bit you a nasty shock. If I get knocked out, the vents on my
shoulder release a cloud of neurotoxins (to which I've been immunized). 

And cyberimplants should be salvageable, so you can *really*
loot bodies when you loot bodies. This suggests using called
shots to avoid shooting up the implants you want, a skill to
remove equipment from corpses, and a black market cyberdoc you
need to establish relations with in order to use your salvage.

Cybergoths: Subculturalists with a suite of "vampire" implants.

Drugs. I can't imagine cyberpunk without drugs. They could give
temporary positive effect, and then some more longterm negative
side effect. [-- Milesss]
You can have it as potions, pills, injections, etc. but I think
the most cyberpunkish and cool options is to have them in form
of derms and inhalers. [-- The Sheep]

"Realistic" gory violence, no just ticking off hit points. Nice
descriptions would be good.

Dealers? Why not vending machines? Or convenience stores? "Yeah, I'll
take a pack of smokes, a fifth of vodka, a 6-pack of uppers, and some
bubble gum." Then walk next door to the weapons shop. <g> [-- Auric__]

Insane AIs with eccentric behavior, not just "take over the world" stuff

Industrial espionage

Conspiracy

Cross-Cultural exchange follows from multinational entities in
charge. Japanese still works, but it would be more current to
include Chinese and Indian influences in the post-industrial
world of tomorrow.

Urban Spawl + Urban Highrise: A BIG city to play in (plus possible
trips to other cities, orbital stations, possibly lunar or martian
colonies, if you want -- but a single city ought to be room enough).
Lots of corporate buildings to visit/raid/apply for a job at.
Socializing areas (bars, clubs, thrill sport arenas, whatever).
Public transit (subway, elevated train, whatever) to get to specific
locations in the huge city (while you can also wander through random
city if you want).

Part 2: Source material

Games:
Shadowrun (published RPG books and supplements, spin-off
fiction, fan material on web, possibly the CCG contains
some useful ideas; also console games) Note that Shadowrun
is a hybrid of cyberpunk with high fantasy.
Deus Ex series
System Shock (1+2)
Anachronox
Fallout 1+2
GURPS Cyberpunk
Cyberpunk 2020 (R.Talsorian Games)
Cyberspace (ICE [Iron Crown Enterprises])
Uplink (http://www.uplink.co.uk/)
Neuromancer (DOS, abadonware)
Rise of the Dragon (ditto)
Syndicate (DOS)

Literature:
Neuromancer (Gibson)
Count Zero (Gibson)
Mona Lisa Overdrive (Gibson)
Idoru (Gibson)
Snow Crash (Stephenson)
The Diamond Age (Stephenson)
Tekwar series ("Shatner", later TV show, computer game[s])
Altered Carbon (Richard Morgan)
Broken Angels (Richard Morgan)
Woken Furies (Richard Morgan)
"A Good Old-Fashioned Future" (Bruce Sterling)
"Heavy Weather" (Bruce Sterling)
Manga:
 Ghost in the Shell
 GITS 2 - Man-machine interface
 Appleseed, Black Magic
 Battle Angel Alita, GUNNM - Last Order, Ashen Victor
 Blame!, Noise, Biomega
 Syrius Scars
 Eden
 Red Eyes

Movies:
Blade Runner
Minority Report
The Fifth Element
The Matrix
Tron
Robocop (and sequels)
Avalon
Ghost in the Shell
Animatrix
Tetsuo 2: Body Hammer
Serial Experiments Lain
Bubblegum Crisis
Akira
Johnny Mnemonic
Brazil
Hardware

And, of course, wikipedia and Google will allow one to
find many more items of source material.

Part 3: The "Cyberpunk Alphabet"
Here's a start on brainstorming identity for your encounters.

A: automobile, accountant, ambulance, armored vehicle, airplane, android, AI,
admin(istrator), alien, anarchist, activist, avatar
B: bus, bioweapon, businessman, beancounter, bartender, beggar, bodyguard,
bounty hunter, bureaucrat
C: child, cat, car, computer, cyborg, cop, cleaner, cyber*, coder
D: dog, drone, data, doctor, decker, driver
E: employee, engineer, executive, enforcer
F: firewall, fireman, firetruck, fanatic, Free Software (rebel AI), file, fixer
G: gang member, guard, gyrocopter
H: hovercar, helicopter, hacker, hologram, hitman
I: ICE, infant, investigator
J: janitor
K: "kriminal", killer
L: lawyer
M: man, mech, manager, motorcycle, medical worker, military, mime, mercenary, mechanic
N: nanotech, nerd, ninja
O: organic tech, office worker
P: pirate, policeman, psychic, pimp, police vehicle, prostitute, punk, program,
politician
Q: queer
R: robot, reporter, rebel, rat, Rastafarian, root
S: soldier, spy, salesman, sex worker, sniper, scientist, secretary, security, spam,
server, samurai
T: truck, trojan horse, terrorist, tank, technician/tech/techie
U: unlicensed pharmaceuticals distributor
V: virus, van, vehicle
W: woman, worm, worker, wizard
X: xenoform (alien)
Y: yesman, yuppie
Z: zoo animal