| I was doing some things awhile back with critical hits, and this thread about wounds got me thinking. How about the hit that takes you below 0 is considered a critical hit based on the House Rule I've included below?
I like the idea of a critical hit to be more rare (as some have suggested). Either needing to exceed AC by 10, or by having to "comfirm" a critical hit (meaning you've rolled the 20, now roll to hit again - success means crit).
Critical hits do not have extra/max damage, you roll damage normally, but they affect your maximum hit points. On the character sheet you'll have 2 scores one is Healthy MAX HP and the Current MAX HP. Current MAX HP does not affect you bloodied or healing surge values.
Crits heal on there own 1d6+CON Mod hp per day. A cleric would have another Class feature - something like this below:
Cure Critical Wounds
Cleric Feature
You utter a simple prayer as divine light washes over your target, healing their most grievous wounds.
Daily ✦ Divine, Healing
Standard Action Close burst 5 (10 at 11th level, 15 at 21st level)
Target: You or one ally
Effect: The target is healed 1d6 + Wisdom modifier of critical hit points.
Increase critical hit points healed to 2d6 + Wisdom modifier at 11th level and 3d6 + Wisdom modifier at 21st level. | 
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