Western games
I look for the following things in Western games:
-Faithfulness to the mood and tropes of the pulp/movie genre, with gritty heroes, fallen heroines, plucky settlers, and perils in every direction. Who doesn't want to see ranchers fight farmers, railroads and the telegraph begin to transform the landscape, and the cavalry ride in? There is often a moralistic code, but that depends on what the Game Master and group prefer.
-Rules for firearms and duels, gambling, and overland (horseback and rail) and riverboat travel. Natural hazards, animals, and trade/barter are also useful.
-Some enlightenment toward women, religious minorities, and aboriginal and enslaved peoples -- we ourselves are in the 21st century, after all. This means not too much gratuitous sex, violence by or against Native Americans and slaves, or prohibitions against female adventurers (of which there were several in the real world). That's not to say that there are no brothels; charismatic, idealistic, or insane preachers; or nasty folk of all colors, it's just that I wouldn't want to rely too heavily on real 19th century stereotypes. See recent incarnations of "Zorro" on screen or in the comics for examples where white men aren't all good or Spanish/Indians all bad.
-Example characters, locations, and scenarios: While Western literature, TV, and film are rich with examples, some guidance on how to convert them into useful gaming writeups would be helpful.
I particularly like "Boot Hill," the D20 "Sidewinder Reloaded," and GURPS "Old West." Most steampunk/Victorian/pulp cliffhanger games have some treatment of the Old West: "Deadlands" uses horror and alternate history, "Aces & Eights" and "Castle Falkenstein" have a card-based mechanic (the latter with fantasy flair), and "Etherscope" and other D20 steampunk games add weird technology and magic.
Of course, Westerns have influenced everything from superheroes (Jonah Hex) and space opera ("Star Trek," "Star Wars," "Firefly") to samurai movies ("Magnificent Seven/Seven Samurai" -- yes, I know the latter came first, but Kurasawa acknowledged Westerns as inspiration), so odds are that you already know how to strap on your pistol and spurs! Saddle up, and happy gaming!