"Stack intellect." All you had to do was ask any question about holy paladins in Wrath, and you'd receive that infamous answer. Intellect was the solution to all of our problems, and it was through intellect that we conquered our corner of the healing game. Intellect allowed us to cast indefinitely without ever running our mana wells dry, and intellect allowed us to cast our most powerful heal without even a moment's pause. It was a matter of fact. Holy paladins would simply overheal in massive amounts, and nobody cared.
I suppose intellect was only half of the problem. Holy Light clearly scaled too well as our gear improved. It was meant to be expensive but powerful, a necessary evil for dire situations. Once the scare of running out of mana was debunked, there was no reason not to use the most potent tool in our arsenal. While the developers may still blame the imbalances in Icecrown Citadel on the extra tiers of gear they had to add to accommodate heroic raid encounters, when an errant DPS class' performance scaled out of control, they were quickly reined in. Holy paladins' power was left unchecked because we were merely healers. The hammer would drop on us in Cataclysm, anyway.