Inspired by this WoWInsider post.
The expansion introduced an array of new races, but none of them quite as wind-chimey as the Naaru. These massively powerful floating energy beings were introduced as a manifestation of "The Light": i.e. the power that paladins (and priests) call upon to heal wounds, smite their foes, and &%@*ing bubble when you were just about to kill them. A humungous one, A'dal, floats in the center of Shattrath, while another, M'uru, is currently being held captive by the Blood Elves in order to steal his power to fuel the Blood Knights.*
At first glance, the Naaru seem like a stereotypical Good Guy force. They are manifestations of The Light, for god's sake. They are supposed to be the benevolent faction of "good" bent against the Burning Legion's rampage of "evil", lending their power to those who would fight the Legion. But perhaps all is not quite as it seems.
The Horde-only quest line revolving around the weakened Naaru K'ure, who is currently trapped inside the crashed Draenei ship Oshu'gun in Nagrand (you know, that giant diamond with all the Consortium and Voidwalkers around it?), along with recent revelations about what happens to M'uru when players attack him in Sunwell Plateu, seem to point to a more sinister agenda with the Naaru.
You see, the Naaru life cycle is such that they alternate between two states: Light and Void. When a Light Naaru runs out of steam, they revert to a Void Naaru, which, get this: sucks up souls to regain its power and return to the powerful Light state.
Think about that for a second. That means that every time you cast a seal or heal as a paladin, you are expending the stolen energy of another person's soul for your own ends. Don't feel so "holy" now, do ya?
The Naaru aren't helping us because they are naturally giving or benevolent, or even because they simply want allies against the Legion. They are helping us because if the Legion wipes us out, their food source is gone. They are helping us so the can use our souls to recharge their batteries later. We probably look like walking chicken nuggets to them.
That cathedral in Stormwind? A monument to worship a race that eats us.
*[SPOILER ALERT]: According to reports from the PTR, in 2.4 M'uru will be kidnaped from his kidnappers by Kael'thas who, after his defeat at the hands of your guild in Tempest Keep, plans to use the Naaru in conjunction with the Sunwell to open a portal large enough for Kil'jaeden (general of the Burning Legion) to enter Azeroth and finally conquer our asses. Having lost their source of power, the Blood Elf paladins head to Shattrath and sign up with the Shattered Sun Offensive against Kael, gaining the true blessing of the Light from A'dal in the process.
A'dal alludes to a prophecy that predicted the Blood Elves would eventually redeem themselves as a race and return to the fold, and explains that this is why M'uru allowed himself to be captured by the Elves.
What, did they really think they could hold down an unwilling demigod? With just three guys in robes shooting beams at it? Pu-leez.