Dr. Street (that's his secret identity -- shh, don't tell anyone) offers details on paladin healing and mana cost, why certain abilities like Holy Radiance were nerfed, and the critical chance of Holy Shock. It seems that paladin healing is intended to be balanced around its many mana-discounted or outright free effects. While if a paladin and another healer both just stood there and cast their cheapest heal, the paladin would indeed run out of mana sooner, the design of the class is intended so that they won't do that, so design has to be balanced around those free heals. I won't pretend to be an expert, but the premise seems logical enough.
Ghostcrawler also mentions specifically that the Holy Radiance and Holy Shock spells are being looked at and could be adjusted if they're underperforming. You'll find the entire post reproduced here after the break, so you paladin healers (or any healers who are interested) can take a look and see what you think.