^ This.

I've raided (casually, but raided nonetheless) since BC and I've gone to the vanilla raids with smaller groups, and I've never felt the encounters to be anything less than epic.

Open space and grandiose settings, no matter how empty of actual players, have always felt epic to me.

I do agree that some of the newer raid encounters are less epic, but for me, what truly makes them epic is the design of the encounter itself. A boss that just sits there until you attack, without saying nor doing anything except taking and dishing out damage, is not very epic.

Herald Volazj always struck me as unnerving and dangerous due to the whispers before and during the fight. Ingvar the Plunderer's fight, even, is pretty epic - he's talking to an agent of Arthas before the fight, and gets transformed mid-way through.

Scourgelord Tyrannus is my current favourite - I'm almost sorry to have to kill the guy. He's instantly present when you enter, and throughout the instance. Once you get to him, he feels like a true nemesis, since he's been dogging your steps up until the actual encounter.

From what other people have said, too, it seems to me that the most memorable encounters just aren't the ones where it's pull-pew-pew-loot. Interesting mechanics, lore material and challenge are what make the fight epic, not the number of people.

Kel'thuzad always felt incredibly epic to me due to the lead-up to the fight. Thinking that it's 10 people locked in his throne room, surrounded by scores of undead... that just felt claustrophobic and terrifying, which is perfect for that boss. And of course, having an animated pile of dragon bones as a guard dog was pretty awesome, too.