Over to the left you can find a link to Frank & K's Dungeonomicon, one of the cleverest (and brilliantly witty) attempts to make sense out of the D&D world that I've ever seen.
I took a different route. I started playing with 1st Ed, and so when I wanted to fix the glaring weirdness in 3rd Ed, I went backwards instead of forwards (not to imply there wasn't plenty of weirdness there, too). The most important thing I took from the old edition was the rareness of levels. In the good old days, only one out of ten thousand was fit to be an adventurer. Everybody else was a peasant, largely indistinguishable from a piece of furniture, or at best a low-level hack "incapable of progressing upwards." Seriously, you could hire those guys to be your soldiers. It said so, right in the book.
I never figured out how those guys knew they could never get more levels, or for that matter, how they got their levels in the first place. But I always felt sorry for them. How much would it suck to be that guy? And of course, the peasants, the ordinary people, who existed solely so the heroes could save them from being eaten by monsters. You could hire an army - in fact, as a Cleric or Fighter, you got one for free - but it was useless. All those zero level dudes could do was die when the monsters looked at them.
Then, being me, I started wondering what effect that would have on society. How could you have a feudal, medieval world with teleporting magic and flaming swords? How would people actually live in a world where the King could walk away from an airplane crash..twice? Just think what that does to the time-honored tradition of regicide.
I was never content to just kill the dragon. I wanted to know why the dragon was there, and what possible good it could get out of a pile of gold, and why adventurers who could magically summon food, clothing, and everything else they needed would even care that there was a dragon eating people. As you can imagine, this was a trial and a tribulation to my game-masters, but they suffered nobly.
Now I have an answer... tael. D&D always implicitly assumed that XP = Gold, anyway, and once I decided to go ahead and make it explicit, lots of things just fell into place. Why do Kings defend peasants? Why do mages sell magic items? Why do Gods empower priests, and pay attention to what mortals do? Why are some people super-heroes, and others mere peons? Why is technology still stuck in the middle ages even though elves are thousands of years old?
Check out the World of Prime worldbook for the answers. Or join my hero, Christopher Sinclair, as he finds out the answers the hard way in my novel "Sword of the Bright Lady."