Casualclysm by a mile.
Wasn't so bad really. A 3 day a week guild could clear everything in the game. All you needed were good leaders and a decent coder for boss abilities. We often raided with 35~ and brought in peoples alts for more warm bodies. Time consumption really came into play with TBC.-Forgot what a time sink the game was, rewarded no skill, the philosophy back then was, play a lot and get rewarded. That's it.
In fact, it's a good thing Vanilla was fairly simple. If you could get a group of geared, not handicapped raiders you would kill bosses and progress. It's not all that different from 10 man normal currently. The problem is heroic raids/modes have elevated the bar. I'd of rather the bar never been raised. Then we'd all be clearing content casually and there would be no hardcore endless debate.