Since paladins rely less on Spirit as a mana-regeneration stat, we have to address them in other ways. We don’t want to change Illumination or Replenishment. However, we are going to increase the healing penalty on Divine Plea from 20% to 50%. Divine Plea was originally intended to help Protection and Retribution paladins stay full on mana. It should be a decision for Holy paladins, not something that is automatically used every cooldown.
I am fine with Divine Plea being nerfed. It isn't necessary for current content if you bring a full complement of 7-8 healers. You have to mix in more FoLs and Sacred Shields, but I think once paladins get accustomed to life without Divine Plea, they'll realize it was just a crutch.
That being said, I don't think this nerf is strong enough to stop paladins from using Divine Plea every cooldown. We use Divine Plea to chain Holy Lights, which has absurd amounts of overheal. Something on the order of 60-70% overheal is common. Even a 50% reduction is not going to be much of a barrier to Holy Light.
However, it's very easy to go too far, and flip Divine Plea into the "never use" category. For example, if it reduced the effectiveness of your heals by 100%, Holy would never use it, as it would be far too dangerous. Fifteen seconds of no heals is an eternity.
What I would suggest for Divine Plea is:
You gain 25% of your total mana over 15 sec. Casting a healing spell will end this effect.
(Where healing spells include Holy Light, Flash of Light, Holy Shock, Lay on Hands, but not Sacred Shield, Seal or Judgment of Light, or Divine Storm.)
This doesn't interfere with Protection or Retribution. For Holy, Divine Plea becomes a skill move. A good Holy paladin can time the ability right to maximize regen for specific fights. If you are just spamming, you can't use it. However, if you use Divine Plea, and it turns out you misjudged, you can resume healing at full strength without seriously hurting your raid. It's less dangerous, and thus paladins will be more likely to experiment with it.
I think this "robustness of failure" is important for a healer. I don't want to wipe just because I made a mistake with Divine Plea.
As well, I think the class designers can play with this variant of Divine Plea a bit more. It's not as binary as the current version. For example, consider:
Glyph of Divine Shock
Casting Holy Shock no longer ends your Divine Plea.
Usable? Maybe. At the very least it is more interesting, and Divine Plea is no longer an automatic decision, but rather an ability that you want to time for maximum effectiveness.
Edit: Added Divine Storm to the list of non-healing spells to address a concern in the comments.