Many words have been spilt over the picaresque nature of this novel, so I won't blather too much about that today. Instead, I'd rather concentrate on two elements that seem far more uncommon in D&D today, but which I believe should be of critical interest.
First, there is the comedic element, which is a peculiar mix of whimsy and satire that emphasizes on bawdiness and trickery. This is common amongst picaresques from antiquity (the Satyricon) to the modern day (Flashman), but seem to have fallen by the wayside in how the game is played. This strikes me as an unfortunate turn of events, as elements of this type of humor are so pervasive in the source material, from Clark Ashton Smith to Fritz Leiber and most obviously in Vance.
Vance cuts deeper, however, with his cynical views on religion, virtue, charity, beauty, and man's capacity for self-delusion. It is here where many games, including my own, lack. We are so busy building worlds that we forget to underline any meaningful statements, or (in the case of story games) we swing too far in the other direction, ham-handedly belaboring a point until the entire group is sick of it.
Happily, Vance provides us a solution to this "all or nothing" mentality that seems to cleave the gaming audience. He never strays overlong into any specific territory, and even the most involved satires never comprise more than a single episode ("The Pilgrims"). Moreover, Cugel is often invited to act badly, with opportunities for mischief dangling like low-hanging fruit. Often these "opportunities" end up doing our hero more harm than good, and we enjoy watching our morally bankrupt protagonist have the tables turned on him. It would do more referees well to learn from Vance, as the plot threads we lay out commonly appeal to the common good, or are reciprocal in nature, with PCs and patrons both standing to gain.
The lie of Dragonlance and its ilk has trained many referees (myself included) to believe that D&D is a story about morality, where right action is rewarded and villainy is punished. This has even been passed along to our bastard child, the video game RPG, which will sometimes offer amoral choices but often have serious consequences for doing so. Many scripted D&D adventures go even a step further, with the only opportunities for adventure being those that coincide with the cause of "good". Yet, the primary sources quoted by Gygax indicate otherwise, and our plot hooks should reflect that. Let's consider some of the "hooks" laid for Cugel:
* A merchant proposes to Cugel that he should rob the home of a wizard while he is away.
* On passing, a man explains how he has toiled for forty years to earn a great treasure. He now only needs to go collect it.
* A deposed king explains to Cugel how he has been searching for the artifact which will restore him to the throne. Moments later, Cugel stumbles upon the artifact laying on the beach.
* Monsters capture Cugel, but offer to set him free should he attract others to the cave where they dwell.
* A clan of yokels offer Cugel safe passage through a section of forest, but want his female companion in exchange.
* The mayor of a small town offers Cugel a position that will give him access to the village's entire savings.
... The list goes on. In each of these circumstances, there is clear opportunity to profit by taking the low road. Complications arise, of course, but the author never passes judgment and neither should we.
The complications I mentioned previously are another important aspect to understanding the humor of Vance - the swindle. Generally speaking, everyone Cugel encounters is out for themselves on the Dying Earth an they care little that it comes at the expense of others. The majority of the characters are actively trying to screw one another, and those who act righteously are hypocrites, fools, or both. Referees intent on emulating Vance should consider this and have a stock of scams on hand which NPCs can employ in their attempts to hoodwink the players.
The second aspect of Eyes of the Overworld which I want to discuss is the dramatic reversals of fortune that Cugel undergoes over the course of the book. One day a king, the next enslaved, this is common amongst picaresques but exceedingly unusual in D&D, at least until the dice turn against you. There is a hard-coded upward trajectory in D&D where one gathers treasure, gains experience, and grow increasingly tougher to kill, insulating oneself from bad fortune. The natural "arc" is one of a successful self-made man, ala Robert E. Howard's Conan.
Amongst old-school circles there has been a push against this trend to a certain degree, with greater rewards for carousing and otherwise pissing away your money. Compared to the highs and lows of Cugel, however, this is nothing.
I suspect that there are two reasons why this has developed:
1. "My Precious Character": Canny players discovered a long time ago that referees, wanting their players to have fun, will bend over backwards when serious pressure is applied to them. Whining, guilt trips, arguments, and the like are the surest way to get many a referee to comply with their demands. Many a die has been fudged and many a point-buy system introduced to ensure that the players get their way. This also extends into play itself, where the arc of accumulation and improvement is all but guaranteed.
2. "My Precious Campaign": Give the players too much power, the story goes, and they'll run roughshod over your campaign world, murdering important NPCs, turning every dungeon into a cakewalk, and ruining all your plans. Before you cry too many tears for that imaginary noble patron, however, you should keep in mind a lesson that the earliest referees kept close to heart: what the DM gives, the DM can taketh away. Which leads us to...
Monty Haul or the Boom-Bust Cycle of Gygax & Ward
Rust Monsters, disenchanters, and the Tomb of Horrors all have one thing in common - they are corrective measures, ensuring that players never stay ahead of the game for two long. In the first two examples, they do so by stripping the characters of items which have proven to become burdensome. In the last, it is to strip the players of characters which have become problematic. Later philosophies would mock these devices as attempts to correct the mistakes of the referees, but I'm not certain that is the case. Temporarily giving players access to powerful magics and great wealth acts as a "release valve" where they can blow off some steam and throw their weight around, lording it over their foes until the wind blows the other direction, leaving them destitute (and likely with a great number of enemies looking for revenge). While players may initially chafe at such vagaries of fate, I suspect this is just a question of managing expectations - hand them a crown, or a dragon mount, or a rare artifact at low level, and let them wreck a little havoc. Then, just as quickly, snatch it away. The consequences of what they did with such power will likely create more adventures then you would've conceived on your own