Don't get me wrong, I like threat. The whole idea about threat is really interesting and a good one. But it is designed around two premises that just don't practically work. So theoretically I definitely see why Blizzard ever thought threat was a good idea and why they've decided to keep it for as long as they have. The idea behind threat is A) that there should be a teamwork between players and B) that the tank should have something to do. Looking at the first premise we can see that Blizzard did hand most dpsers (sorry warriors) some sort of tool that would allow them to help tanks with their threating, either it be tank targeted skills like MD and TotT or self targeted skills like FD, Fade or Cower. Because Blizzard initially dreamt about how players would assist eachother in attaining their common goal, how they would struggle in trying to make the experience as pleasurable and easy as possible for everyone involved. *cough* Unfortunately we all know that's not how the vast majority of random pug experiences has turned out, and they never will. If you ever read any of my guides regarding tanking you'll know that my motto as a tank is "the group is the enemy, not the mobs". As a tank you struggle against the other players rather than collaborate with them. Everything you do revolves around how you can trick your fellow group player into making tanking less of a hassle for you. I'll admit that I, in some sort of masochistic way, enjoy this part of tanking. Probably because it keeps tanking challenging. Unlike with healing and dpsing, you can't really outgear instances in the same way as a tank. The challenge depends mostly on the skills of your group. So threat can be fun. But that is only as long as it goes well - as soon you end up in a group where the dpsers only minimally outgear you, you'll have hell. Sometimes I'm even happy that I am the one dealing the most damage in the group, because it means keeping aggro is so easy. It's also a major reason to why I enjoy lowbie tanking so much - threat is no problem at all.
The tools for reducing threat aren't that great either. You could give everyone a Feign Death (and then watch no one use it, as with hunters), but that would remove the whole issue about threat anyway. If everyone could just swipe their threat clean, then why even bother with threat in the first place? Skills like Fade that reduce threat temporarily are very similar to this and also make threat overall kind of moot. Or you could give players a threat reducing mechanic like Cower and Feint, and although I think these skills actually have the most interesting mechanic, by forcing the player to keep an eye on their threat constantly instead of just swiping it clean when they over-aggro, these skills have one huge problem. They're worthless when you need them and you don't need them when they're useful. Cower reduces threat by 10%, but the only time when threat is an issue is usually very early on in a fight. Early on you don't have much threat anyway (just more than the tank) and 10% from barely nothing is nothing. Early on you might get aggro from 2-3 skills, from being good on threat to suddenly having aggro. Cower won't save you. Later on in the fight when you might have a big chunk of threat, the tank is usually so far ahead of you anyway that you don't need to reduce your threat by 10%.
Looking at the second premise we all know that most tanks think that threat is a necessary evil. Much as Blizzard said in their post - no tank wishes to gear for threat and do it only as much as absolutely necessary, because survivability is so much more important. But I understand that they're afraid to make tanking too easy and boring. I will admit that there are plenty of raid bosses that are dreadfully boring to tank, especially if we look at the normal modes. Some bosses you sleep through, like Magmaw and Atramedes. Some bosses require a minimal effort of target switching, like Halfus, V&T, Council, Al'Akir, Nefarian (unless you're the add tank in phase 3), Chimaeron, oh my god the list just goes on. I haven't tanked in FL yet so I'm unsure about that, it seems like a similar mix of challenge and dull staring into a crotch as t11 was. Most of the time, honestly, it's not the tanking part that is challenging, it is surviving all the other things that everyone is trying to avoid that is challenging. As a tank I have to mind the fire and stand in the right place as much as the other guy. I have to think about making the boss face in the right direction as much as the raid has to think about not standing in front of it. And furthermore, threat only has two phases in raiding anyway - the problem phase early on in the fight, where some dpsers might snatch aggro and get insta-gibbed, or the non-problem phase, where after a couple of target switching the tank is so much ahead of threat that no dpser has to worry about it anymore (as mentioned above). So threat in raids is already not working in a good way - it's either too much of a problem or no problem at all.
I've said it before too, there is no other role that can be locked out of its own reason to exist in the way a tank can. If I join a group and everyone in the group has aggro but me, for any reason, it makes me rather obsolete. Imagine if you somehow could keep the healer from healing or the dpsers from dpsing? If I end up in a group where no one lets me pull or where I just can't keep aggro I either tell them I might as well go dps, or leave the group. Because I'm not tanking. And unlike with healers and dps, my failure might have nothing to do with skills, but plain old lack of gear. There is nothing I can do on my ilvl 340 tank against Loves threat, and it is equally annoying for both him and me.
So the argument to have threat because it makes tanking more interesting doesn't really work either (as Blizzard also said). It only makes the fight more "interesting" very early on, and does nothing really later on. So what could make the fight more interesting and challenging to a tank throughout the fight instead of just the first 30 seconds? Survivability. As a healer you notice at once the major difference between a tank that knows how to use cooldowns and one that doesn't. Giving the tank more power over their own survivability would definitely make tanking a lot more interesting, and definitely sort between the good and bad tanks in another way. I think the only reason they haven't done this already is because it could make tanks too good. It could make some tanks into one-man armies (hello DKs), because most tanks do good damage and have some sort of self-heal already, increasing their ability to survive by themselves might do them overpowered in solo-pve and/or pvp. But it would make pve-tanking so much more fun. It seems now Blizzard are thinking in the same lines and might implement this kind of skills later on.
Right now threat is iffy. Nothing can annoy me more than just not being able to keep aggro, regardless of what I do. My Taunt button goes hot, and after a while I just can't be bothered anylonger. I can only imagine the amount of players who've given up on tanking because the whole idea of threating seemed too overwhelming to them, the whole idea of having to fight every player in the group just didn't sound very appealing (gee, I wonder why?). Tanking shouldn't be about the struggle for threat, it's about doing nice pulls, locking down the right mobs, leading the group through an instance in a smooth way and staying alive while taking a beating. Right now, threat is designed to be regarded as part of a boss mechanic, part of the fight and that everyone has to work together to overcome. This is very rarely what actually happens, and I am glad that Blizzard have finally realized this (although I would've really wished for threat to work, because like I said, as a mechanic it's an interesting one). Instead, everyone is frustrated at this pink elephant in the room that no one wants to acknowledge but it's there and you have to get around it somehow.
Will it make tanking too easy? This is really what I was dreading when I initially read about the change. But then I thought - is keeping aggro the only tricky thing about tanking? Definitely not, as I wrote above there are tons of things to do as a tank already, and most of the time I've thought of threat as something frustrating rather than fun. I'm happy when I keep aggro of course, but I'd be equally happy if I could unload all my focus onto using my survivabililty skills at the right time, moving around in the right way, not standing in shit, keeping the pace through an instance and knowing that the dpsers can fully focus on just doing dps (which they were anyway, but still). As a tank we definitely have way more things to think about already than the average dpser/healer (although that isn't always true in raids). The change hasn't made tanking EZ-mode, as a tank I'll still have to combat the ninja-pullers, the random afking healer and the no-ccing dps as much as before.