Gladiator's Resolve allows Protection Warriors to switch to DPS without changing their gear (well, unless they really want to maximize the best secondary stats.) It's an interesting experiment and one that allowed for an unprecedented playstyle - a melee dps that uses a shield.
But it's going away. The quasi-spec was certainly an interesting idea, but I always suspected that they would either have to give Warriors a fourth spec to allow it (something they only gave Druids out of sheer necessity) or they would have to give all tanks some kind of dps capability. They've opted to just remove it, which will upset a lot of people I'm sure.
Converting a tank spec to a DPS one consists of two challenges - buffing damage and reducing survivability.
Tanks already do damage enemies in order to generate threat, and as a result, tank damage needs to be balanced at a level that is proportional to DPS damage. Thus, it's not that big a challenge to simply add a coefficient when in "DPS mode" to make a tank spec work as DPS. (Ok, it's obviously more complex than that, but not substantially different from balancing between DPS specs.)
There's also the need to scale back some of the survivability. After all, if a tank-as-dps spec could take just as much punishment as a tank-as-tank spec, why would you ever take the latter? Or normal melee DPS?
But honestly I think that the main reason behind this is more about a return to fundamentals. They're doing this with all specs, trying to focus on the core of the spec's identity, and having this kind of dangling DPS quasi-spec off to the side for Protection was a bit odd.
Sort of in a similar vein, Mistweaver Monks will no longer be able to heal through their melee strikes.
Looking at Mistweavers in a vacuum, this makes tons of sense. If a healer can heal sufficiently while also contributing as much DPS as a tank or even a bit less, it clearly has a big edge over other healers. Essentially, you're in this place where balance is impossible. You either become far too overpowered - healing just as well as other healers but also contributing DPS, and thus making all other healers obsolete - or your healing is handicapped to make room for the benefit your damage brings. In a raid with four or five healers, that might be ok, because you could split the difference and be the half-healer within your raid's roster of four-and-a-half. But in smaller content, particularly 5-player dungeons, where you are the only healer, how can you justify taking up that spot when you're going to be struggling to keep everyone up thanks to your handicapped heals?
The solution in Warlords was to give Mistweavers two stances - one to serve as a dedicated, classic healer standing in the back and tossing out healing spells, and the other to focus on "Fistweaving," which you could opt into if your group or raid didn't need quite as much healing.
But with this "starting again from the basics" approach, Blizzard has decided to simply make Mistweavers into straight-up healers. However, this isn't really getting back to how things were, because Mistweavers were originally designed specifically to be the "punch-to-heal" spec. In fact, I think this is a large contributor to people playing the spec (it was what finally got me to try out leveling a healer, though to be fair she never quite made it to the level cap, sitting at 88.)
Still, perhaps it's for the best. After all, a healing/dps hybrid spec is inherently kind of problematic.
What was that? Discipline Priests?
So Discipline got an interesting mechanic starting, if I remember correctly, in Cataclysm. They could blast enemies with Smite and Holy Fire (finally giving those spells a use) and thus heal the party. It gave Discipline something to do if group/raid damage wasn't that much of a threat.
It was this Atonement effect that I believe really inspired the design of the Mistweaver Monk (which of course came in the following expansion.)
But Discipline's theme was more around absorption shields. Power Word: Shield was fundamental to the spec, and finally gave it a way to shine after the dominance of Holy in early WoW.
The problem is that those shields were way too powerful. Shield spells are inherently kind of over-powered. As long as the target is taking any significant damage, the shield will always be useful, and on top of that, until that shield breaks and the target starts taking real damage, everyone else's heals are useless.
So Discipline has been overpowered - partially thanks to our over-reliance on healing meters - and so Blizzard wants to move Disco priests away from that as their primary theme.
But once you take a holy-themed, cloth-wearing healer away from the one kind of heal that the other healing spec for the class doesn't use, you kind of paint yourself into a corner.
So Discipline is going whole-hog on this hybrid DPS/Healer thing. With all the problems I just described for Mistweavers.
As someone who doesn't really play healers, it obviously doesn't affect me that much, but I do really wonder how well this will work out, and if Blizzard might be forced to reconsider Disco again (yes, I like to call the spec disco.) They do seem to be designing this hybrid status as mandatory, which might make it somewhat easier to balance than Mistweavers. Still, it remains to be seen.
Honestly, I think this all would have been a lot easier if in 2004 they had made Priests like every other healing class, giving them one healing spec and then making Discipline into a kind of Inquisitor-type, casting holy spells from afar, with maybe more of a direct-damage theme counter to Shadow's DoT theme.
But that was 11 years ago. Then again, they did make Survival into a melee spec.