The answers are all pretty good (I've put them all behind the break -- thanks, Twitterers!). From what most people said, lore and story play a huge part in making a fight feel epic, and not just the story of the game, but the story of the guild playing as well -- when your guild almost wipes, and pulls off a great kill, that's an epic battle whether it's in Karazhan or in Sunwell Plateau. Challenge also helps -- people raid to be pushed to the edge of their endgame abilities, and when a well-tuned fight does that, it's one of the best experiences in the game.
Great answers all -- hopefully we'll get to see more epic encounters than not in the Wrath of the Lich King (as Matticus points out, we don't need 40 or even 25 players to have a truly epic encounter with Arthas). And if you haven't started following our Twitter account (@WoWInsider) yet, please do, and stay tuned for more interesting and informal Twitter polls like this.