The developers have been answering some lore questions. You can find the full thread here. Now, one bit about the Light caught my eye...
Q: Can you please explain how "light" works? The lore states that undead are physically incapable of using the light, much like the Broken, but then we have Forsaken players casting healing spells, and Sir Zeliek in Naxxramas using pseudo-paladin abilities.
A: Without spoiling too much, we can tell you that wielding the Light is a matter of having willpower or faith in one's own ability to do it. That's why there are evil paladins (for example, the Scarlet Crusade and Arthas before he took up Frostmourne). For the undead (and Forsaken), this requires such a great deal of willpower that it is exceedingly rare, especially since it is self-destructive. When undead channel the Light, it feels (to them) as if their entire bodies are being consumed in righteous fire. Forsaken healed by the Light (whether the healer is Forsaken or not) are effectively cauterized by the effect: sure, the wound is healed, but the healing effect is cripplingly painful. Thus, Forsaken priests are beings of unwavering willpower; Forsaken (and death knight) tanks suffer nobly when they have priest and paladin healers in the group; and Sir Zeliek REALLY hates himself.
Okay, so why is this interesting to me? Well, I heal on four toons, and I do have backstories and motivations and all that for all of them (I may not RP in game but I love storytelling and enjoy the lore). And so I've decided to figure out how all of my toons would be affected by this and what the implications might be....
Of course, Lyllea is a night elf, and she doesn't think of herself as wielding the Light. She thinks that she's calling on the power of Elune to heal. So that leads to the question... if she's not using the Light to heal, would her heals still be painful to a DK? That could go one of two ways. Either all positive energy is painful (which would mean that all heals are painful, no matter who casts them), or only healing done by the Light causes pain (which would mean that Lyllea's heals or any other heals cast by a night elf would not be painful). Now Lyllea is generally a bit of a soft touch, but death knights creep her out. That entire "Suffer Well" thing is just not her cup of tea. She's all about preventing suffering which is why she's a discipline priest. The idea that she could be causing pain to someone when trying to prevent harm, well, that's going to throw her for a loop. If her heals hurt DKs, then well, she'll only heal them as a last resort. If she can get someone else who won't hurt them to heal them, that's what she'll do. She may not like them, but she has no desire to cause them further pain.
Adowa on the other hand, does heal with the power of the Light, and she's likely to be far more practical about it. After all, being healed's got to be less painful than dying, which is the other option. Plus, life involves pain. If you don't want to experience pain, you should look into dying.
Monera (my druid) would think "well, death knights are unnatural, so of course the power of nature would hurt them. Oh well, stinks to be you," and she'd keep throwing her hots around.
Eiunn (the baby shaman) would likely be coming at it from a different perspective. Shamans (from what I've seen) use the elemental power of water to heal, so it really wouldn't make any sense to her that her heals would cause pain. She'd probably do a great deal of soul-searching, and ask death knights before she healed them.
And my DK? Well, brain damage does come in handy occasionally. When you're more or less perpetually happy, pain doesn't bother you that much. And while she used to wield the Light and heal (or at least she thinks that's what she did before she died), she's accepted that the Light doesn't like her very much anymore. That's why she goes around helping people whenever possible, she wants the Light to like her again.
I think the entire idea of the Light hurting undead does bring up some very interesting questions. In addition to the ones above, how bout these...
Belf paladins. Before their redemption, when they were siphoning off power from that Naaru, did their heals hurt?
Can Forsaken priests heal with the power of the Shadow instead of the Light?
Do belf priests heal with the Light, or do they call on something else?
Isn't the Light itself more of a human/dwarf concept than anything else? If so, could the pain caused from wielding or being healed by the Light only apply to races that believe in the Light, sort of a 'I believe it will hurt me therefore it will' thing?
Honestly, I think you could go many different ways on this, depending on how you wanted to interpret things. I really like the idea of it all being in the head, so to speak. It leads to some interesting religious discussions. If I believe I'm wielding the power of Elune to heal you and you believe I'm using the Light, which one of us is right? Are we both right? Does it matter?