Once upon a time MMORPGs had a holy trinity of tanks, healers, and damage dealers. Most games had some pure classes, that is tanks which were highly resistant to damage but couldn't deal much of it, healers who could heal a lot of damage but couldn't deal much of it, and dps classes who did nearly all of the damage. And then there were hybrid classes, who did two or even three roles, but worse than the pure classes.
World of Warcraft, like previous MMORPGs, started with that same model. Only after a while the hybrid classes complained, because if you're even a little worse in tanking or healing than a pure class, you won't get invited to raids. Thus Blizzard made paladins and druids tank as good as warriors, and heal as good as priests, in some situations even better than the pure classes. Now the pure classes started to complain, because playing a warrior or a priest was understood to be a deal: You would tank or heal best, but give up on your damage dealing for that, thus hurting yourself in soloing. Blizzard responded by increasing the damage of the pure tanking and healing classes, and by introducing dual spec, making it easy for them, and the hybrids, to switch from a tanking or healing role to a damage role. The pure tanks and healers had effectively been transformed into hybrids as well.
And then came patch 3.3 and Blizzard introduced the Dungeon Finder. The math of the Dungeon Finder is simple: If you take away the lost time for assembling a group and traveling to the dungeon, the xp and other rewards gained per hour in a dungeon are suddenly superior to anything you can do solo. The more dungeons you run, the faster you advance. And all those hybrid classes, both the original hybrids and the previously pure tanks and healers, quickly found that if they queued up for the Dungeon Finder in their tank or healing role they'd find groups much, much faster than if they queued up only as dps.
The ones left behind in all this are the pure dps classes. They didn't gain much from the introduction of dual spec, except for being able to change the flavor of their damage. They didn't become hybrids, they are the only single-purpose classes left. That was all nice and dandy when soloing was in thing to do, because damage is the most useful ability when soloing anyway. But while their hybrid friends are already looting the end boss, the pure dps are still waiting in that Dungeon Finder queue. And even if they make it into a group, they aren't necessarily dealing more damage than a hybrid who signed up as dps. Playing a pure dps class today, especially one without AoE damage, feels like shooting yourself in the foot. Everybody else is as good as you in your single role, without having to suffer from the consequences of a single-role class.
Makes me wonder if Blizzard is stopping their continual re-invention of the holy trinity here, or whether we will see some move that turns the pure dps classes into hybrids as well. Mages that can heal? Rogues that can tank? Sounds strange, but if anyone had told you in 2005 that paladins would one day be one of the top damage dealers that would have sounded strange to you too back then. Will we all be hybrids one day?