I know, Blizzard always makes the same comment that you shouldn't take one ability out of the context of a class' whole kit, but when you compare healer masteries, it seems like they fall in 3 categories.
1: Requires some management to gain value
2: Flat healing increase; only useful if not overhealing
3: Extra healing is useful, even on overheals
(Druid's Harmony is a buff that's not difficult to maintain, but you do have to pay some attention.)
Paladin mastery is in a group of its own, as one that is almost always useful healing. Illuminated healing mitigates the consequences of overhealing, which reduces the depth and challenge of the class. I don't heal heroics right now, but most fights in Flex I can spam the same 4 button rotation (J, HR, HS, LoD) for the entire fight, without even watching who I'm casting on, and match every other healer. Paladin mastery feels overpowered right now, but more than a nerf, I really think it needs to be replaced with something that doesn't simplify the class.
So, the less important part of this post, what I'd like mastery to be. Beacon of Light is a core part of Paladin gameplay, so I would like to see a mastery that improves the healing transferred to Beacon. At end tier levels and stacking mastery, Beacon would heal for ~100% of direct heals and 33% of AoE. Direct heals on the Beacon would either have ~25% increased effect, or "splash" to another target or several targets.