How much can you tinker with D&D before it becomes something that works great for writing stories but not for actually gaming? I've been mulling over various campaign ideas of late, and thinking of the elements that make D&D game play D&D game play. Here are three elements (I'm sure people can name more, or even refine these) that I think are essential to keeping in D&D no matter what sort of campaign world one creates.
1) Emphasis on dungeon environments. This is where D&D started. Whether a red sun burns in the sky, there are three moons, or the only available character classes are fighting men and Wookies, the traditional game environment is in the dungeon. Naturally, this one element takes on the most importance, so the other elements have to support it. This isn't to say that other adventure environments shouldn't be had from time to time, but if the available game options are not conducive to dungeon play then you don't have a viable D&D game (more on this below).
2) Questing for treasure. Without the quest for riches, why the hell else are the characters down in the dank underworld risking their lives? While various other motivations can and should be incorporated into dungeon play, the base line should be that adventuring in the underworld has the potential for awarding treasure. As a consequence, treasure and ruins should be scattered throughout the world.
3) Enough toys to keep things interesting. People like toys. Even if your game world has less magic or even no traditional magic items, players still need toys to keep them interested. So magic items don't exist? Then instead of the wand of fire balls there better be a blaster pistol somewhere, or a laser grenade.
Since I'm assuming dungeon play is the primary setting, all things must support it. Sometimes people forget that D&D is a game. Yes, it is a role playing game, but that doesn't mean it should be devoid of game elements that really only make sense because it is a game. One could come up with an elaborate fantasy world, with rich cultures, interesting races, a deep history....but the big question that must always be asked is...what is happening in the dungeon? Have the monsters been so cut back that there isn't anything interesting to kill in the dungeons? Are the challenges and rewards making the adventures interesting?
In the end, the greatest danger in world design, IMO, is in creating the backdrop forgreat stories, but horribly boring game sessions. Worlds should be designed from the dungeon up, so to speak, not so much from the big picture down. Or at the very least, from the dungeon up, flavored by what you have in mind with the big backdrop.