Agree with Devant. As long as Ret Pallies have fewer troubles than other classes/specs, please fix the others first. I'm a little tired of the Pally mania, they're doing fine if not spectacular in every aspect of the game.
But if you need to change Pallies...
Get rid of Divine Shield (keep Divine Protection and let it also reduce stun/interrupt/pushback duration by 50%)
Let holy shock hit five people
Make Judgment of Justice reduce movement speed by an additional 30%
Give Pallies their own Anti-Magic Shell (identical to Death Knights)
Create a talent that adds Retribution aura damage to weapon strikes or makes it an active damage aura while keeping the nerfs noted above (however you can balance this to increase overall damage while reducing burst)

There, now Paladins can aoe heal, aren't unstoppable PvP juggernauts (or at least, less) and Ret Pally burst is weakened but damage slightly increased and kitability also reduced. No Pally QQs about Prot these days, and as one myself I know that there's no reason to.
This would all seem very easy to do.