I've been thinking recently about why I never feel like running games in the world of Krynn or the world of Forgotten Realms, and when I do, I resort to Killing Elminster or other cheap tricks, which don't buy into the setting. Even when I follow the basic advice of using the things that will immediately hook PCs, I tend to dislike it. And yet, my favourite setting to play in is Shadowrun - despite it's giant buckets of setting. I assume this is because the setting is disjoint from the actual play of RPGs.
Whenever I try to put players into the Forgotten Realms, I feel like there's too many people who are already doing thing things players should do. Which is GREAT fiction and tone setting - but kind of terrible RPG design. I feel like the slots my players should be occuping are filled by people who are explicitly designed to be more powerful than them (or, more aptly, more developed along the paths). This is even true of Krynn, which is a setting where people began running away from hobgolins and killing shit with frying pans, but then the heroes grew and became greater. And then, overshadowed the PCs. The greatest story inside Krynn is the story already told.
The worst single game I've ever seen for this is, of course, Vampire the Masquerade. The larger game affects the smaller game, and so the players, who are supposed to be on the bottom of the stack, and raising their station - firstly, can't do that with breaking the sole law that governs their kind, and secondly, trying to take a position such as Prince through guile and cunning. Succeed at ANYTHING that will get you power, and the next teir of the setting will destory you. This is basically the most frustrating setting I've ever encountered (and, the only one I am going to outrightly call badly written, Dragonlance and Forgotten Realms feel like great settings for novels).
Shadowrun is awesome, because the novels, the stories, the plot, are all about the weird machinations between Megacorporations and Dragons and Immortal Elves - and every time something happens, NO ONE GIVES A FRAG. Some fraggin' wizworm fragging some corp slot isn't going to change some barrens rat's life. Not going to make the world different for them. The drekhead in charge is different - but that's ok. And, of course, that's what the game is ABOUT. These people being Shadowrunners who attempt to frag the system, and if they system is boring and uncaring - that's better, not worse.
So - I am currently running Darksun, and I've decided FUCK THE NOVELS. I want to make sure I've no idea who saves Athas from itself - the only being who may save Athas is my PCs. And if they fail - the Dragon Kings win. Let's make the world beautiful and wonderful.
The setting of the game should be either smaller than the PCs, or larger than the PCs (according to the theory I currently hold). Meaning that the setting is so much larger than their changing it is hopeless, and the space for play is within the bounds created by it. The important bit here (which is where Masquerade fails) is that the space for play must be clearly sketched as being inside those lines. Or the setting should be small enough, it becomes the board on which the character actuall play - for example playing the characters of Amber who are so almighty they literally shape the world at a thought.