Alright clearly there are, and have been, issues revolving around Power Word: Shield for quite some time. It's either too efficient, or not efficient enough. This toggles between it costing to much or too little mana, or the value that it absorbs is either too high or too low.
In previous blizzard posts they've stated that they want to ensure that while playing a discipline priest you still feel inclined to heal opposed to just shielding.
I agree with that, but I DO NOT feel that making shields unsustainable through the use of resource limitation is a good way to do this.
Personally, I love rolling shields on as many people as possible. It's an act of predictive damage prevention rather than the conventional means of reactive damage correction.
Example: Pre-shielding stacked raid members on Maloriak during his Red Vial phase due to incoming AOE damage, Chimaeron's Fued phase, etc.
While direct healing (Hots, Aoe, Single target heals) is rewarding as you're healing someone for straight health which is both fun and rewarding, it's also rewarding to know that your effectively assisting someone in mitigating damage and increasing their survivability.
There have been many valid and good suggestions about changes to Power Word: Shield such as changing how weakened soul works.
In my mind the Discipline Priest acts as much as a damage preventer, as they do a damage corrector (healing). My suggestion to keep discipline fun is to require discipline priests to both shield, and heal, in parallel with each other.
Right now, our shields absorb quite a substantial amount of damage. When a target is shielded, based on the rate of change of their health, you effectively know they are "safe" for up to 30 seconds with your shield on them as it absorbs all damage taken up to a limit.
To counteract this, I think power word shield should be mechanically reworked to "Reduce damage taken by x% for a specific time duration" - Much like Pain Suppression works right now, where x is affected by our mastery rating, and the time duration is lower than that of the current implementation. This methodology was used to correct issues surrounding Power Word: Barrier in Wrath.
At medium to low health, priests are forced to heal their targets prior to rolling shields, due to the fact that damage will not be mitigated by 100%, and their target is still susceptible to damage and death.
At medium to high health, the priest can safely use encounter triggers and knowledge to ensure shields are up at times of high incoming damage, to reduce the amount taken, rather than to absorb it completely. It provides the monitoring of shield durations, and timing with event mechanics and should play as an interesting "rotation".
Changing the mechanic in this way will than also cause abilities such as Prayer of Mending to trigger when shielded targets take damage, and thus encourage use and glyphing of POM which is a known concern.
Ok, but what about Pain Suppression?
This is a cooldown, and I believe it should be a powerful spell. Can it switch to absorb 100% damage for a time duration? Of course not, we might as well call it "Divine Shield" if that were the case.
My suggestions in this thread are primarily to address concerns surrounding Power Word: Shield opposed to Pain Suppression. But now that I'm on the topic, my suggestions would either be to simply increase the damage absorption percent on pain suppression (don't forget PS still has other mechanics associated with it), or make pain suppression act as a synergy to the priests shields effectively increasing the effectiveness (absorption %) of the next pw:s cast by the priest, while mimicking the already existing mechanics within Pain Suppression.
I like rolling shields, preventing damage should be baseline for a discipline priest.
Power Word: Shield needs not an increase in mana cost, but mechanic change to absorb a % of damage rather than 100% of a fixed number.
Pain Suppression should be adjusted to account for this change causing 2 similar abilities.