Okay, I've started posting about mechanics, so all that leaves is the actual game world. If I didn't offend anyone while talking about mechanics, I'm sure I'll offend someone while talking about game worlds. It's easy to get emotionally invested in a game world, and I think that's important to a certain extent. In any case, I'm just going to talk a little bit about my own views regarding the purpose of a game world and probably offend all the White Wolf fans in the process.
In my opinion, there's no point in making a game world that isn't deeply informed and shaped by firsthand experiences or that doesn't help you express or resolve things that have happened in your own life. In other words, the best fantasy game worlds aren't about fantasy or escape...they're about shedding light on reality. I'm not going to say that there's anyone who hasn't done enough to need to resolve their life through the lens of a game world, but there's definitely a worldly experience factor that makes game worlds better or worse. And I'm not going to say that teenage minds produce any less ambiguity and dilemma than mature minds. Regardless of who's making it, the success or failure of a game world will naturally depend on how many people can identify with it.
For instance, look at Lewis' fantasy world (meaning that it's essentially a game world without a mechanic to interact with it). It's a paradise for theists who are dissatisfied with the Bible, but like its underlying premises. Naturally people like that will gravitate to a world where there are clearly delineated authority figures, the savior figures dies and returns, good and evil are writ large (and clearly delineated), and there's essentially no question of who's going to win in the big picture (regardless of the struggle in the meantime). It's essentially an attempt to pick and choose among the stories of the Bible and combine them in a way that's both accessible and coherent.
Now consider the world of Tolkien, another major theist. Evil is elemental, but not in the same way as Lewis' evil. Lewis gives us a top-down evil, where the source of elemental evil is corrupting its way down to us. Tolkien's evil is bottom up, springing mainly from small sources (pride, envy, etc. or even a mere lack of constant moral consciousness), and the small quandaries and moral dilemmas of daily life can slowly turn normal people into figures of elemental evil. Like Lewis' world, Tolkien's world is also easy for us to identify with since it squares with our actual experiences (normal people, ourselves included, can become monsters without having the devil appear and turn them into monsters). Also, the magnitude and severity of the events in Tolkien's world, and the smallness of so many of the central characters who are drawn into those events strikes a chord with many people who experience the same thing in their own lives (for instance, I'm writing this from Anbar province, and I can definitely say I identify more with Tolkien's world now than I did before).
Let me give you another example of a game world that a lot of people identify with...White Wolf's World of Darkness. Let me just say up front that I loathe the World of Darkness. I think it's just about the most poorly thought out game world you're likely to encounter, despite all the fluff, expansions, and reimaginings it's undergone. I mean, this game world has no substance to it. It's a mile wide and an inch deep. The World of Darkness, Vampire especially, is just a power fantasy...there's no depth or history. Sure, there's the suggestion of history and the suggestion of wisdom, but there's nothing there (the game itself is a perfect analogy for being a teenager). You're only really playing in one of two eras (modern or medieval), so you never have to hit the books to learn about history. The lack of history is written off as "well, that's not the way it really happened. We know what was really going on -wink- -wink-." You have immensely powerful creatures that essentially do nothing with their power but possess it, hold themselves aloof from society, and engage in petty sniping amongst themselves.
To be honest, you could make an equally good game (I would argue a much better game) that stresses all the same points and has the same cathartic value by creating a Lord of the Flies game. Actually, Lord of the Flies would be more like a Vampire the Dark Ages game. A Vampire the Masquerade analog would be more about a bunch of kids staying home without a babysitter (or being complicit with the babysitter) and having candy for dinner. And if you ever attend a session of live action vampire the masquerade, you'll see that this is really all it is. So now that I've horribly offended all the Vampire players, let me qualify some of what I'm saying.
I'm not going to say that the World of Darkness isn't a world you can identify with. Obviously, it is. I'm not going to say that there's nothing you can personally gain from playing games in the World of Darkness...loads of people find it fulfilling. However, I will say that the World of Darkness generally can't deliver the intrigue, insight, or darkness that it claims to (except in the form of personal interactions between the players...not the characters).
I don't think the fundamental concept of the game world is bad...I think it's brilliant. I just don't think the game world that lies in the binding is the same world they offer you on the cover. I mean, the purpose of a fictional monster is usually to shed light on our conscious or subconscious fears (fear of being eaten alive, fear of death, fear of water, fear of foreigners, fear of sex, fear of the afterlife, fear of a total lack of an afterlife, etc.). I think if you're going to make a game world targeted at teens where the premises are: the players are monsters the monsters feed on humans the monsters have to keep their existence a secret or perish from the earth the monsters are hunted by the handful of humans that know about them the monsters are reviled by other types of monsters who are essentially in the same boat
The conclusion should not be: the monsters are the ones secretly ruling the world the monsters are totally cool the monsters control humans as puppets
I mean, take Vampire, easily the most popular White Wolf game. I'd start with the basic fears of teens and build the whole game world around ensuring that the characters are subject to those fears: (e.g., fear of humiliation, fear of loneliness, fear of own immaturity, etc.). Make it so that feeding off of humans causes intense pain instead of this ridiculous "swooning embrace" or however it's described. Make the existence of vampires a widely known fact throughout global governments, who use them as puppets (either through bribery or blackmail). Cut down White Wolf's enormous vampire population to a realistic predator-prey relationship, so you almost never meet other vampires. How about hitting the ultimate teen fear, and make it so that vampires look normal, but are naturally repulsive to the opposite sex? I suppose that takes you pretty far away from the traditional vampire, but the traditional vampire was imagined to resolve a totally different demographic's set of fears. In any case, it's really not a game world I need to make, but I'd like to see someone take a stab at it.
Werewolf is a similar deal. You could probably recapture a lot of the original fear and horror of the werewolf by not giving players control over their werewolf form (crinos, or whatever the hell it's called). They go to sleep as a wolf or a human, then they wake up covered in blood. It's on them to find out what they did.
Using my criteria for a game world, I think the best thing to come out of White Wolf's World of Darkness so far has been Changeling the Dreaming, which is like instant teenage catharsis. Yeah, you're a strange creature, and most people can't perceive your rich inner life. There are plenty of people who get you, but you have to go out and find them. Also, it puts a lot of stress on figuring out who you really are. Also, I liked the fact that White Wolf's big character sorter for the different types of fae had some unmistakable corollaries in the cliques you see in every high school (trolls, sidhe, satyrs, etc.). That's something Vampire was sorely lacking. And while Vampire stressed the wars and cutthroat political dealings between the clans, Changeling was more about the different types of fairies having to form a functioning society together (mainly broken down only by Seelie/Unseelie, which basically meant nice people versus assholes). I know this sentence is going to win me some enemies, but I think if you play Changeling in grade school or high school, you'll end up a more functional person than if you play Vampire (probably with less social and sexual dysfunctions). Okay vampire fans, let me have it...I know I'm asking for it with that one.
In any case, I haven't really touched on some of the traditional dungeons and dragons game worlds yet, and I don't know that I really have to. By now you know my take on the purpose of a game world, and I know there are a lot of people who disagree. Plenty of gamers view the primary purpose of game worlds as escapism, not therapy. If you're one of those gamers, all you really need to know is which fantastic elements you want in your game world and then find a way to coherently combine them (or incoherently combine them...definitely looking at the makers of Shadowrun here).
I will say this though...if you view the primary purpose of a game world as escapism, you should be wary of including too much weird, fantastical stuff for one reason. It will quickly get mundane, and your world will instantly lose its wonder. There are worlds where alchemy can seem like making a can of Campbell's soup, and there are worlds where making a can of Campbell's soup can seem like alchemy (thinking of a wilderness survival game in D20 where getting supplies and eating them became an adventure in and of itself). If escapism is your goal, you should always start small and establish an idea of normalcy before you introduce the weirdness. If you're really artful, you can create a world where the players don't even take mundane things for granted, and a totally non-magical game world can give you an escapist thrill.
What I'm trying to say with all of this is that it's essential that at some point you make your own game world. This post is really just an attempt to get people discussing what it takes to make a game world that satisfies them.