IMHO the problem of PW:S is its lack of CD. Yeah... I know YOU don't like what I'm saying. But this is how I see it:
If PW:S is an expensive spell you aren't gonna use it because it's better to cast other spells, except to proc Rapture... and if you need to spam it at a certain moment you get OOM.
If PW:S is a cheap spell you are going to spam it and forget the other 12 spells you got.
I think PW:S should be a cheap spell but with a 5 secs CD by default, lowered to 3 secs CD by a discipline talent which would also give you the ability to use a spell that would allow you to cast without any mana reduction as many PW:Ss as you want for X seconds, for instance 10. This would help you a lot to prevent damage in certain situations but wouldn't allow the healer to just spam shields during the whole encounter. This ability should have a 2-3 min CD so you could use it 2-3 times/boss.
What do you think? Mb Blizzard don't want to give us another CD, but I really think this would be the best they could do to solve the PW:S problem. Had anyone already purposed it??
Excuse my poor english.