Dungeons, not usually the dinky prisons beneath castles that most people think of when they hear the word; but our own version of massive underworld fortress. We spend a lot of time exploring them and exterminating their denizens (and taking their stuff). Sometimes they are simply the lairs of evil creatures from which they raid human settlements, sometimes they are ancient ruined cities of the dwarves that have been overrun by the forces of evil*, sometimes they just are.
D is for Dragons. The ultimate arch-nemesis of the game, they sit on vast hoards of wealth for reasons only they can fathom. I like that EGG threw a bone to the far east with the invention of good dragons, but I can honestly say I have never used one that I recall, for any reason. I use dragons in my campaign as the objects of a great quest, and they are always evil. Blame it on the Hobbit (cartoon, then book) and the cover of my Holmes basic set; those two sources covered how I perceive dragon-kind more than anything else, and the supporting sources of Beowulf and Dragonslayer are pretty similar. When I DM I use them pretty sparingly, they are tough opponents that you meet at the end of a quest, over using them makes them commonplace and ruins the mystique.
Dungeons and Dragons is the name of the game after all.
D is for Dwarves. This is turning into a pretty Tolkien heavy post, Dwarves are always modeled after Tolkien in standard fantasy. We even changed the spelling of their plural from Dwarfs to Dwarves because of him. The Dwarves in the Hobbit and Gimli in the Lord of the Rings set the standard. They also are responsible for the majority of subterranean complexes in my Garnia campaign world. The only time I have ever seen them as different from the Tolkien paradigm was in my Roman themed campaign where they were the magical mutant male children of human families and considered the sons of the god Vulcan; and even there they kept some of their racial mythology as consummate craftsmen, particularly with metal.
D is Demons and Devils both, because they form the supernatural arm of the bad-guy leadership and some of their toughest shock troops. My Garnia campaign is very much oriented as a good versus evil conflict with a serious multi-planar invasion aspect. Demons and Devils are the invaders. The princes are super opponents that move largely behind the scenes and their underlings make great end boss leader types for lesser evil creatures. That said, in all my years of DMing, I have used them sparingly, like dragons familiarity breeds contempt.
D is for Dice, and man do we love our dice. We love our dice so much that we will seek out the coolest, prettiest or most unusual dice that we can find just so we can hoard them like dragons. I personally prefer my dice like my swords, simple and functional, but I appear to be largely alone on this stance. I implemented an anti-cheating measure back when I was in junior-high that said essentially that I need to be able to read your die from where I sit without any trouble. That cut down on the uninked (and uncrayoned) dice at my table and the later ones with all the decorative inking that makes them impossible to read.
Then we have our weird dice rituals and superstitions. No one may touch my dice. Everyone must touch my dice. Dice that behave poorly must be punished somehow. I have seen bad dice thrown across the room, put in a time out (in a separate box or in the freezer), and executed (crushed by a hammer or thrown in the fire) with or with out the other dice present as witnesses.
We also have our dice rolling procedures. Mine are less stringent than some, more than others. I require a reroll of all leaners, those dice not resting flat on the table for whatever reason and I force a reroll of dice that land on the floor. If miniatures are in use, you must not roll into the combat area where the miniatures are. Some people require the use of a dice cup or a dice tower, I just will expel obvious cheaters. Darryl C. used to have a cheating method of rolling multiple dice one after another so he could see if the first die was good and, if not, knock it to another face with his next die; so I had to make all rolls of multiple dice simultaneous. My son John also forced me to reiterate this rule when he decided it would be fun to roll his dice with great ceremony one at a time so it took forever to make a character.
D is for Delftwood, the SCA barony in which I live. My wife and I have been less active in the SCA for several years now, but we are trying to return to a more active status now that the kids are older. We used to be pretty involved there. I have served in the past in various official capacities. I was a deputy herald and a deputy knight marshal. I was the heavy weapons champion one year. I ran a second fighter practice in the north with my friend (and distant cousin) Kevin M. for about 5 years. We put on a regional fighter practice (and potluck picnic) every Memorial Day for 5 years running and I have autocratted several events. My wife also served as Chatelaine for a few years. We were even asked to run for Baron and Baroness once, but declined to. I love Delftwood and it's people, and I miss being more active there. Delftwood is also the home of Aethelmearc's first king, Sir Yngvar the dismal, who along with Sir Dain Ferris trained me as a fighter. Dain did most of the training, and he trained Yngvar, so I guess he deserves most of the credit :) I'd also like to give a shout out to Baron Rian who influenced my glaive style so greatly.
D is for Dad, specifically my dad, who was one of the first people to play D&D with me when I was a kid despite the fact that he is not a fan of the fantasy genre. He stuck it out and ran a party by himself through the Caves of Chaos while I learned to DM. He DMed games at my junior high D&D club. He bought me my AD&D Players Handbook and DMs Guide. He bought me a set of Grenadier D&D miniatures as a souvenir from his trip to California back in '82.
*Yeah, Moria. Professor Tolkien invented the megadungeon, deal with it.