Elfs and Donjon are what really got me thinking, deeply, over what exactly a role-playing game is, and how it works.
Coming from an old-school hack-and-slash RPG background, it really blew my mind when I realized that players shouldn't just sit back and rely on the GM to force-feed them the story -- no, they should get off their damn butts and MAKE THE STORY THEMSELVES!
(And hanging out here on the Forge has also taught me how important a social contract, and expectations in general are, but that's a whole 'nother thread...)
(Edit) What the heck!? This was supposed to go here: http://www.indie-rpgs.com/viewtopic.php?t=12913&highlight=