Lately, I've begun to see some posts saying that the spell system in WoW is breaking down at the very high end. I think that there is some truth to the matter. Here is my description of the problem:
The amount of damage done or healed by a spell or ability is given by:
Total = Base + Modifiers
During levels 1-59, the Base damage is greater than Modifiers (+dmg/heal), so getting new ranks of spells is important, as it's the only way to significantly increase your damage.
At level 60, the Modifiers start becoming larger than the Base. Eventually, you can stack enough modifiers so that the Base is negligible compared to Modifiers. At this point it effectively does not matter what rank of spell you use, as the Total damage is determined mostly by the modifiers.
Total = Modifiers
However, the cost of the spell is still determined by the Base spell. So you choose the cheapest spell you can cast. Your cost is significantly reduced, but your damage is still roughly the same.
The second problem is that there are ways to reduce the cost even more: Spell Crit, via Illumination; and Mana/5s. With enough of these stats, you can reduce your costs to zero, while still keeping about the same total damage.
In effect, the old paradigm of 'Significantly larger effects require significantly larger costs' no longer really holds at the very high end of Warcraft, and this is game-breaking.
Honestly, the best solution would be to dissuade casters from casting lower ranked spells, probably by nerfing the effect of Modifiers such as +heal on lower ranked spells. If a lower rank of a spell costs 50% less mana, it should heal about 50% less damage.
This isn't a problem specific to paladins, but the existence of Illumination and a really cheap Flash of Light makes it very visible when looking at our heals.