Everyone else will probably be commenting on it in greater detail. My take is that it is a reasonable experiment. Maybe it will work, maybe it won't. Also, the single best comment on this system comes from Martinel, posted on WoW Insider:
Well. Rogues, mages, warlocks, and hunters...being pure DPS classes, they won't really be eligible for this, will they?
I guess you could call this a hybrid tax...
But seriously, how would you solve the tank problem for real? The truth is that, in every trinity MMO I've played, save one, the tank has been the limiting factor.
Maybe it would be instructive to look at the one game that was different: Age of Conan. To me, Age of Conan tanking seemed pretty hard, so it was quite surprising that you could find tanks.
After ruminating on it, I've come to the conclusion that the key difference was that Age of Conan required two tanks. At first this seems counter-intuitive, how can requiring more tanks for a harder job make it easier to find tanks?
But players have always cited the responsibility of tanking as a key factor in keeping people from tanking. Perhaps people are more willing to tank if they know that the responsibility will be shared, that there will be another tank in the group to help you.
And it's not really that hard to have another tank in the mix. You tank X, I tank Skull. You get the boss, I get the adds. I have better gear, so I'll tank the boss and you can watch and learn from me and do a little DPS. It's easier in instances than raids, because it's really not worth wasting time swapping out a tank for a single fight if you will just need to get them back again.
Healing could also benefit from having two healers, depending on how you heal. If healing is a little more passive, so the healers aren't constantly stepping on each others toes. I'm less sure of this, because AoC required two healers, but healers were pretty rare.
So maybe the proper group would be 2 tanks, 2 healers, and 2 DPS. Or maybe instead of DPS, have 2 Crowd Control, and have all three roles put out roughly equivalent damage. And design the class system to match that ratio, rather than having 50%+ DPS classes/specs.