For everyone who is NOT a holy paladin, remember back in the days when you would roll your eyes at critical strike rating on gear? "Oh just give it to the mages, blech." While it was always a matter of personal preference, it took Blizzard a long time to figure out how to make crit worth anything to healers.
The philosophy against crit to which I always adhered before Wrath, was that while a crit heal may be nice, fancy, and powerful, you cannot rely on it. Often many healing crits just get into the realm of overheals, at which point, it's worthless. Not a "waste" necessarily, because it did not cost you anything to get that extra BOOM in your heal, but rather pointless. Except in some situations, you do not get to decide when your heal will crit. A notable exception would be for Holy Pallies with Divine Favor.
If you knew when your heal were going to crit, it would be very different. Maybe I would use a crit Flash Heal over a more expensive, longer casting Greater Heal. But, because I cannot predict when a crit will happen, I cannot afford to only use a flash heal and some crossed fingers to heal up the tank when she's taking a lot of damage. The problem with critical heals is that you cannot rely on them, and therefore when you are healing, you must work on the assumption that you will get no critical heals in order to be safe. Better safe than wipe.
But Blizzard wants us to use crit, and changed our talent trees. Most notably, IMO, for priests was the change in Surge of Light. It used to be that Surge of Light would only give you a free Smite proc, but now they've added flash heal, which makes it incredibly useful. Free and instant cast? I'll take some of that. Even more recently, the changes in Holy Concentration. I preferred the old version that gave us mana back when we overhealed (which in turn made crits less worthless - if you crit and overheal, you got mana back), but regardless, the point still stands. Crit is now worth it. Everything is crit happy. Even our HoT, Renew, can now crit. I stack crit like crazy now.
Yet, I still can't help thinking that the original philosophy remains applicable. If I were to look at the healing done over an entire night, sure, it seems that crit heals are a substantial part of my throughput. But in the heat of the moment, you still can't rely on it. If I pop a glyphed Circle of Healing, I'm almost guaranteed at least one crit, and therefore one free flash heal. But almost guaranteed is still not an actual guarantee. I'm not going to be one of those QQing forum posters that whines all about how OBVIOUSLY no one at Blizz playes a healer class. However, I do wonder how much the developers understand that fundamental philosophy.
Long story short? Crit is nice and fancy, but is still not reliable. Designing a class spec based around a dependence on crit is therefore just asking for trouble.
The hullabaloo about the potential Illumination nerf is a perfect example of that. This post has already gone on too long (even for me), so I will summarize the Illumination nerf arguments, and my thoughts about it, in a later post.