The state of the game world is defined when a player tries to do something. For example, a player might search the room he's in for traps. He sums his Wisdom-equivalent stat and up to one applicable skill, and rolls that many d20s. The DM rolls an amount of d20s representing the local difficulty of finding traps. Any of the player's dice that roll higher than the highest of the DM's dice are counted as "successes" for the player; if there are none, the DM gets successes in the same manner. If the player succeeds in this case, then he did, in fact, find traps; he gets to make as many statements about the traps as he has successes. Failures mean that he didn't find any traps, and the DM gets to make statements about the player's failure. Successes can also be held over as bonus dice for a related roll, such as trying to disarm the traps, etc. Because Donjon often involves rolling many, many dice, RyanRiegel has made an SML program to alleviate the problem. And ChainMaille has also made a [Perl script], which includes such features as a user interface.
DonJon games tend to be very silly. Players get to make up their special abilities (though they do have to be vetted by the DM), so things like "hear what people mean, not what they say", "smashy smashy", and "make magic food" show up as abilities. They also get to make up their own equipment, which has led to the existence of a Vorpal Sledgehammer and a Hairtie of Bishounen-ness. And, of course, they get to make up things about the world around them, with results like locks that can be picked by inserting a fingernail and turning, or goblins with no legs (still bloody dangerous fighters, mind you), or traps with off switches...
RyanRiegel is currently running a campaign. We plan to get others started so that more EastiePeople can be introduced to the fun that is DonJon. If you're interested in playing, ask RyanRiegel for a copy of the PHB or his revision.
Things we have learned in playing DonJon:
Because this should be online somewhere, here are the character generation rules: /DonjonCharacterGeneration
My first (and so far, only) attempt at running DonJon did not go well. The problem was that players kept questioning aspects of the system, asking me for rulings, and then questioning the rulings. Sometimes this would go on for a while before deciding that my ruling was reasonable after all. This killed momentum and made me very annoyed. Admittedly, I wasn't doing a very good idea of keeping momentum, and it's possible that people felt like they had no advantages in-game and therefore tried to gain advantages in the meta-game. Also, combat can be kind of slow, and unless everyone's pretty creative about using their successes, it gets to be a grind. I'll admit to not being very good about using successes. --SeleneTan
"You just took two away from its 'incite orgy' ability!" - JeffBrenion
"It is fetish-on-fetish. Your dirt fetish... is going to resist." - RyanRiegel
After the silver floating cyborg weapon of gay-cyborg-displacer-beast-love-propagation is defeated:
Jeff: "We need some way to shut [the troll] up. Could you find a ball gag?"
Someone: "Provisions check?"
Ryan: "It's kind of... silver. And floating."
ChainMaille: Okay, we need to decide our alignments. I'm chaotic neutral.
Everyone else: You mean chaotic evil!
ChainMaille: Hey! That was not my fault!
ArthurHall: You mugged the mayor! And threw a rock into a crowd!
ChainMaille: That rock was thrown at me first! And the mayor was wearing red; he was asking for it!
According to JoshKline? on Schmack, the game is actually "Don Juan". Perhaps it's for the best.