As too many paragon paths in my list, Messenger of Peace is a fairly impressive option when used as intended, but can become crazy in less conventional builds. In this case, ‘as intended’ implies being a pacifist cleric, one of the most extreme types of character in the game, as it sacrifices most of its damaging capabilities in order to become the ultimate healer. The powers and features in this path share this pacifist theme, in that they deal no damage on their own, and often ask the character to refrain from otherwise hurting his enemies. However, they impose a lot of negative conditions and penalties on foes, making for a pretty strong package.
While all elements of the path are quite potent, it is the 16th feature which justified its inclusion in the ranks of the broken. Called Aura of Peace, it imposes a continuous -2 penalty to attack rolls on all enemies within 2 squares. This is amazingly strong, yet somewhat balanced by the fact that the pacifist cleric himself is a relatively fragile character focused on ranged attacks, making it hard to really exploit such a short range aura. But put it on the hands of any capable melee combatant, and you have the ultimate defender feature!
To put the effect of Aura of Peace in perspective, we can split it in two separate abilities: one to defend oneself against attacks from nearby enemies (-2 penalty to attack rolls against you), and one that protects your allies (-2 penalty to attack rolls against your allies). The first one, by itself, would be one of the best defensive boosts available for a melee character in the game - almost equivalent to a +2 bonus to all defenses, with the right positioning. As for the second part, it is a defender’s mark/aura on steroids (sans punishment), with the added benefit of actually stacking with marks/auras - both yours, and your allies’. Having both effects is way too effective, as well as out of place for a leader option.
As written, Aura of Peace greatly rewards characters who can hold large groups of enemies around them, making it a great fit for defender archetypes, but not so much for its alleged target, the pacifist healer. In addition to lowering the power level of the feature, my proposal also tries to add some pacifist flavor:
Aura of Peace (16th level): Enemies within 2 squares of you take a -2 penalty to attack rolls against bloodied allies.
A penalty that only works against bloodied creatures will be less than half as effective as a constant modifier, particularly given how great healers pacifist clerics are. But it’s still nothing to sneeze at, and can realistically be used without staying at the front line at all times - rather, you can stay on the back until a comrade gets low on health. At that point, either you close in to succor him, or he falls back under your protection. You still have the option of getting into melee with this aura, keeping it mind that it won’t help your own chances of surviving - for that, you’ll have to rely on your Vow of Nonviolence.