After getting a critical effect from your Flash of Light, Holy Shock or Holy Light spell, gives a 100% chance to gain mana equal to the base cost of the spell.
This talent is the heart of the paladin Holy tree. But there are a lot of misconceptions as to what it does, or its real effect on a paladin.
The best way to think about Illumination is that it reduces the average cost of Flash of Light, Holy Shock, or Holy Light by the paladin's spell crit. If you have 10% spell crit, your healing spells are essentially 10% cheaper.
This is a very powerful ability, especially as it is open ended.
Before patch 1.9, Blizzard was very careful to keep Illumination in check. It was very hard to get spell crit for paladins. For example, the quest reward [Eye of the Beast] specifically excluded paladins from using it.
Paladins highly valued items such as [Azuresong Mageblade], because spell crit was much harder to come by for them than it was for the other classes.
Then, in patch 1.9 and the paladin review, Blizzard did an about-face. They decided to embrace spell crit as a "feature" of the paladin class. Other healers went for spirit or mp5, paladin healers would go for spell crit, and that the hallmark of paladin healing would be efficiency. We got talents that increased our spell crit, and our itemization began to feature spell crit.
I think that time has shown that this was a bad decision.
As a result of our new talents and itemization, we have been able to stack spell crit to significant levels. 30% or higher spell crit is not unheard of, and that translates into healing spells which are 30% cheaper, which makes our efficiency insane compared to the other healing classes.
The proposed nerf to Illumination cuts the amount of mana returned in half. So if you have 10% spell crit, your healing spells are on average 5% cheaper.
The problem with Illumination isn't Illumination itself, it's that Blizzard let paladins have too easy access to spell crit. But cutting Illumination in half devalues all the items out there with spell crit. Which is pretty much all our healing gear (PvP, Tier gear).
To illustrate this, look at the paladin Holy tree. We have a talent Sanctified Light, which increases the crit strike chance of Holy Light by 6% for 3 points. That talent effectively reads "The cost of your Holy Light spell is reduced by 6%". Which is a pretty decent talent for 3 points, and more or less in line with other classes' cost reducing talents.
Under the nerf, it reads "The cost of your Holy Light spell is reduced by 3%". Which is not that good. I would say that is not worth 3 talent points. The same thing occurs with Holy Power. 5 points for 5% cost reduction is decent. 5 points for 2.5% cost reduction is not.
Paladin healing endurance needed to be reigned in. But I don't think nerfing Illumination is the correct way to do it. This problem is primarily a problem because of the changes wrought in 1.9, and the resulting itemization. Those are the problems that should be fixed. Hitting Illumination is a band-aid fix, and has many ramifications. One of them is that this change pretty much shatters the top part of the Holy tree.
To be honest, paladin healing needs a proper overhaul. It manages to simultaneously be too good, and not good enough. Once again, I chalk this up to a lack of vision on the part of the Blizzard devs, or a lack of being able to communicate that vision. Watching these changes, you get a real sense that Blizzard doesn't know what it wants the paladin to be, and is just running around slapping band-aids on the class, trying to keep it from breaking.