QFT. This guy knows what it's all about. Agree with everything you said in your post.You know, I think you might be on to something, OP. That it was the design of the raid encounters and what tanks were asked to do, is what made tanking fun. Looking back at the Cataclysm raids, I can list some encounters that I really enjoyed tanking.
Tier 11 was hands down my favorite tier to raid this expansion. Tanks were given a lot to do, whether it be kiting, interrupting, or just plain positioning. As the expansion progressed, the tanking requirements became more and more simplified. Firelands had some interesting mechanics on some fights, but the tanks weren't really engaged as much as they were in T11. Alysrazor was fun to tank, but for different reasons. And then we get to T13, where seemingly the raid devs have just given up on engaging the tanks.
My view might be a bit skewed seeing as I play a Blood DK, which is arguably the most "active" tank currently. I think that's the reason I'm able to put up with the drudgery of Dragon Soul to be honest, because while DS is boring, I still have to pay attention and time my Death Strikes correctly. Even then though, it's still not engaging to me.
I would hope that they step up the raid design in Mists, because most of T12 and all of T13 were completely awful for tanks. I think the design of the raids, and the role of the tanks in the encounters, is what dictates the fun of tanking, more than anything. They can try this "active mitigation" thing, and that may or may not work out (let's hope it does). But the raid designs need to be engaging if tanks are going to enjoy tanking.