We were promised Power Word: Barrier some time ago1 but it ended up being cut. I believe this was because they just weren't getting it to work the way they wanted to. At the time I remember thinking it was a very difficult spell to implement. Obviously one way to implement an AoE shield is just to dump an individual shield on each person in an area, but that doesn't really have the right feel to it.
The implementation they came up with fits the idea of standing under a big shield that you share with your friends. The shield has a certain amount of damage it can absorb in total, but also a limit per hit so that if one person takes a massive hit they don't hog the whole shield.
The problem with this implementation is what happens when you cast it just one person. I'll use level 80 examples because they are more in our familiar range of understanding at this point. If you have around 4000 spell power then the shield can take about 51.6k, with 10.3k being the maximum per hit. If you cast that in front of the boss where the tank is standing then the tank gets 10.3k off every hit for the next five hits before the shield collapses. If a boss is swinging for 25k damage, this is basically Pain Suppression. If a boss is swinging for 35k then it's only 28% damage reduction, but that's an awful lot. What's important, though, is that it is a 52k instant cast heal on a single target in 10.3k installments. Think of it as a stupidly overpowered Renew.
Compare this to its AoE use. Suppose you are fighting the 10 player Sindragosa encounter. Her frost aura does 3750 damage every three seconds - and will be reduced by almost exactly 20% by resists. If non-tanks have an average of three stacks of mystic buffet, and they are gathered up under a shield, that means the shield will be block two ticks of the aura. This is not really that impressive. In fact, considering the positional requirements, it is arguable that Prayer of Mending is equally powerful or more powerful for this application, and that has a 10 second cooldown instead of a 2 minute cooldown.
For single massive AoE blasts there is a good chance that Power Word: Barrier will be inordinately effective. This is because, most likely, each person under the barrier will benefit from the full reduction of the barrier from a single hit even if that total overwhelms the total absorb of the barrier. That is, if your entire 25-player raid is standing under the barrier when the Lich King casts infest, it probably blocks 10.3k from each person, despite the fact that this means it absorbed over four times its maximum amount. This would be consistent with the behaviour of Anti-Magic Field, the spell that Power Word: Barrier was clearly based on. If the code does not have this mistake then it would basically be a 2-minute cooldown double strength Prayer of Healing.
Power Word: Barrier is good for two things: 1) For use as an external damage reduction cooldown on an individual who is standing in an isolated location or who is not standing near anyone else who will be taking damage; and 2) To take the edge off a powerful AoE attack that hits the entire raid when the raid can afford to stand together in preparation for the attack assuming that the way the coding works allows it to absorb far more than is intended.
When they announced Power Word: Barrier I really wasn't sure what kind of mechanics they could use to make it work. Clearly, they want it to have the thematic idea of sharing the shield between the people under it rather than giving each person their own shield. That requires a very large shield to be useful, but they certainly don't want it to be a super single-target shield.
These are some very significant problems to overcome. Unfortunately the design they settled on does not overcome them. The most likely outcome of Power Word: Barrier is that discipline priests will be seen as the external tanking cooldown healer - and will quite possibly be overpowered for this reason.
1. I believe they first announced it for patch 3.1, but it might have been 3.2