If you've been playing this game for long enough, you hopefully have some insight into how this is going to turn out without having to wait for 8.0's launch. It's the same dev team. They know how to make a good product, but everyone has their blind spots.

They've once again started with the assumption that if they implement a design direction for tanking, then players will automatically follow their prescribed approach. History shows us that this isn't the case. Your first question should be: Why should we bring an MT and an OT?

A double MT composition seems like a less likely choice, because counterattack procs are going to be factored into your total damage output. You don't want two MTs competing for procs. But what about double OT? Your damage isn't contingent on counterattack procs. From the slides, it sounds like you have the edge in terms of raidwide mitigation, which is what we really care about at the end of the day. At this point, I'm already questioning why I shouldn't lock out the 'pure tank' slot and just run double OT.

Of course, if you buff those counterattack procs so that they do a lot of damage, guess what we're doing? MTs are going to stand in avoidable damage to gain extra procs and make healers pick up the slack. And if you make MTs defensively powerful enough, then people will drop the second tank and add in an extra DPS.

I'm not saying that it's terrible and the sky is falling, I'm just providing a gentle reminder for the nth time in 10+ years that players rarely do what they are instructed to do.