Recently some holy paladins have been saying that their tree needs to be remade from the ground up. They need new mechanics, new spells, new everything. If history is an indication, they're wrong. Holy can be made to work, or may already work (I've not tried it yet), with the current mechanics or something very close.
Look at prot. Pre-BC is was pretty crappy. It was completely limited by its mana pool, so make a fight longer than 1 minute or two and it stops generating significant aggro. To make matters worse, there was not a prot tanking set. The overall effect was that regardless of the state of the prot tree, they couldn't hold aggro for long and they'd get torn apart in the process.
TBC turned that around completely. What changed? Well sure the tree got buffed, but except for the changes to reckoning and redoubt (which don't affect boss fights much anyway), not a ton changed. Paladins didn't switch to rage either. Instead they got spiritual attunement. This single ability made it so paladins could last. They finally got tanking sets, allowing them to be something other than mana sponges. SoR was buffed significantly, but I'm not sure that it was as big of a deal. Either way, just two or three changes, two of which weren't even anything radically new, transformed the spec.
Now I'm trying out ret and I'm seeing a similar pattern. There were not basic changes to mechanics. We didn't even get a ton of new attacks, pretty much just divine storm. So what do I think fixed it?
AP->SP talent: This isn't entirely new, being a talent that shamans already had. Still, this talent has a huge impact. It effectively means that our heals scale with attack power. It makes damaging judgements (which are now all of them) scale.
Judgements of the Wise: This is sort of new. The regen buff is a new concept in WoW, though clearly born from vampiric touch or mana tide totem. The huge regen for the paladin, that's big. No longer do we have a single mana pool that can be easily worn down, or even more quickly burned. We can keep going, no matter how long the fight lasts.
These two abilities combined to make us into what a lot of people think of us as: warriors with heals. We must now be fought in an entirely different way. Not only do we have multiple health bars, but we also have multiple mana bars. Now stuns must be used as interrupts rather than just a way to piss us off. We still can't interrupt well, but the higher damage from the new attack and from massively buffed judgements means that we can force other hybrids to shift from damage to healing.
I hope holy can get the same treatment, if it hasn't already. Identify the base problems from which all other problems emerge. Fix those with a new spell or two and some tweaking, and there you go, the class, despite the same name and basically same mechanics, is now entirely new.