An interesting discussion the other day got me thinking about the fiction of 4e. Not in terms of novels and the like (though I'm sure those would be an interesting subject) but rather in the fiction implicit in the game. This is not, to my mind, about the little map in the back of the DMG, or even about the idea of Points of Light. Both are good, interesting things, but what I dwell upon is the fiction as implied by the rules. That is to say, what does it mean that these classes, races, items and spells exist in the world, and how does that shape things.
Possibly the most profound change to D&D that 4e brought about was the introduction of symmetry. Characters are all roughly on par with one another in terms of capabilities. There are plenty of differences in the small details and between roles, but by and larger, two characters of the same level are in roughly the same weight class in almost every measure (combat ability, skills, stats, gear and so on).
This was not true of previous editions. The most obvious discrepancy was between spell-casters (especially magic users) and everyone else. At very low levels, the wizard might be a one-use wand, but at higher levels, he could reshape mountains with a single spell. Later editions worked to close this gap some, but it was always a baseline part of the game. If nothing else, the spellcaster was going to be more effective on the round he cast a potent spell than the round he didn't, while the fighter would be pretty consistent in his damage. A similar problem existed for specialist classes (which is to say, thieves), and broadly speaking different classes peaked at different times (all to say nothing of potentially tragically drastic problems of gear disparity).
4e smoothed that all out. Everything's on pretty much equal footing. And that's great for gameplay, but a little rough for fiction. It's a lot easier to build a story (or a world) out of a messy system full of odd discrepancies than it is from one where everything fits together neatly.
This is not to say it can't be done. In fact, I think it's possible to build a great many very interesting fictions around 4e, but the secret of doing so is to recognize that even if the mechanics push things to equity, the setting need not do the same.
The rub is that, as rules material, all races and classes (and to a lesser extent, monsters) are presented on fairly equal footing. When your group sits down to make characters, there is nothing that dictates that Dragonborn Paladins are rare or that Human Fighters are common. Some pairings might be suggested by the mechanics, but all options are equally available and equally valid.
For chargen, this is a great thing, but for fiction, it's problematic. If you infer a setting that is simply a big melting pot of these classes and races, you are inferring a fairly boring, sloppy setting, and in turn, a dull fiction. Fiction depends on tension, which in turn depends on discrepancies. Some races need to be rarer or more common than others. Some classes need to have strong cultural roles, with baggage that varies from place to place.
Consider the Warden. I really dig this class, but there's a strong implicit story to it - if there are Wardens, then there are things they protect. So what happens when you ask how many Wardens there are in a setting? If there are only a few, it might be a secret, elite order, or the remnants of an old tradition, clinging to the past. If there are lots, then there might be a warden for every place of importance in the setting. Those answers and all the answers in between mean a lot to any player who wants to play a Warden and to the game as a whole. And that's just one axis - we haven't even touched upon the role of race in the process.
By making these kinds of decisions, the DM is capable of determining what is anomalous or rare in a game. Perhaps there are only Seven Paladins in all the world at any given time. Perhaps Sword Mages are only trained in one tower off on this mysterious island. Maybe Shardminds exist only in a hidden valley where their worldship crashed. Maybe the Dragonborn have a vast mercantile empire, and can be found everywhere. Maybe there are no Dwarves.
But this does come back to the strength of 4e's level playing field. Historically, if there were only 7 Paladins in the setting, that usually meant that no one could play a Paladin, because those slots were reserved for cool NPCs. By shunting the rarity of things over to the fiction, you can open the door to any character the players want to play, and allow it, with an understanding of how it fits into the world. While not every player looks to embrace the unique or anomalous, many do. Playing the oddball, outcast or outsider is a lot of fun, but it requires that there be something to be outside of in the first place.
It's important to note that none of this is what one might normally think of as world building. It's not about writing histories or developing factions. Rather, it's about arranging the pieces you've been given in a way you (and your table) find satisfying. This means making the world less fair than the game, but that's the only way you're going to make a world worth playing in.