I know I'm several days late on this meme, but my first thought of D&D is always this:
That was my first real intro to D&D, and I still regard as the great single-volume RPG rules compilation ever published. And I will (rubber) knife fight you over it.
Nowadays, the term "D&D" brings a couple of other games to mind, as well:
D&D's gone beyond one company or edition. We're way past that now, I think. It's an interchangable terms not just for AD&D 1e or D&D 4e, but for Pathfinder, Swords & Wizardry, Castles & Crusades, or other spinoffs, retro-clones, and cousins. Each of these is Dungeons & Dragons to someone!
What did my games feel like back in the day? Some of you may not get this, but it was a lot like this:
I'm betting many of your games, did, too, whether you know it or not. A somewhat unwieldy, comfortable, intermittently brilliant, familiar, not-too-balanced, cobbling of rules. It's here, for those that want a closer look.
(EDIT: Had some image link issues. Whoops--should be fixed).