Over at Blessing of Kings, Coriel has a great explanation of how the paladin's to-be-changed Illumination talent -- which has been happily residing in the holy tree since the game's release -- became such a thorny problem.
The talent (as it exists now) returns the base mana cost of a healing spell to the paladin when the healing spell crits. (It costs 5 talent points to bring it to this level of effectiveness and requires 15 points in the talent tree. So a fully maxxed illumination will cost you 20 talent points in the holy tree.) The end result is that, overall, the paladin receives a mana discount on their healing spells equal to their spell crit rate. Prior to patch 1.9, a paladin could boost his or her holy spell crit rate by 5% through talents (priests have a similar talent that boosts holy spell crit), but it was difficult for paladins to get any other spell crit gear. The plate itemization simply didn't exist and there was heavy competition for generic items that a paladin could use like the Azuresong Mageblade.