Tony hit it right on the head. I remember the Heroics in Lich King being challenging for a while, but there were also more people around (guildies, friends, and Puggers) to take on the challenge.

Part of it, I think, is because people were still exploring Northrend after they hit 80. My main had never set foot into Storm Peaks, Icecrown, or Grizzly Hills when he hit the level cap. Come to think of it, I'm pretty sure he was still in Zul'Drak. And this was at launch, when none of us had heirlooms yet.

So there was a lot of content for a level 80 to explore, never mind rep grinds. Consequently I was playing a lot, as were most of my guild, and throwing together a dungeon run was usually as easy as saying "/g Anyone want to run the Daily Heroic?"

Contrast that with Cata, where I hit 85 in a month and was halfway through the Twilight Highlands. I hadn't been to Wash yet, but Uldum was done and Deepholm was halfway done. My Alchemy was maxed out and there just wasn't that much left to do.

So really quickly I was on just long enough to do some dailies and maybe a random dungeon. With all of my guildmates in the same boat (or already working on alts) there just never seemed to be a lot of us around to put together a guild run of anything.

At first I thought it was just a problem with me and my friends, but looking here and at the Forums I see it's more widespread.