I have to say I don't actually particularly care one way or another about the threat change. It was a sliiiiiight issue for some of the DPS in our raid without a real threat dump, but between my healer, my tank, and my hunter, I've never cared that much about threat. It's just been a sort of base-level mechanic that I don't devote much thought to. And even for the dedicated tanks, "doing good tank DPS" is still a worthwhile goal (especially for Alysrazor!), so it's not as if this changes very much.
I am much more interested in how the active mitigation model will work out. On the face of it I think it sounds awesome. I know that, when I'm tanking, I basically keep shield block on cooldown and use shield wall/last stand/enraged regen for spikes. Once those are down I pretty much hope I don't die.
So if they do it right, I really think the active mit model could not only be more fun for tanks, but also be an excellent way for the skilled tanks to differentiate themselves from the not-so-skilled.
Basically when I'm a healer, I think I show skill when everything goes pearshaped and I keep everyone (or enough people) alive. As a tank, skill has always been in keeping things from pearshaping. This change could really bring that skill to the forefront.