Actually, while it's mostly ignored for now, NPCs do have a system like that! It's a bit more complex, though; they have a trust level, but also a fear level, a value level, and an anger level. Value's the only one that needs explaining I think; it represents how much of a benefit having you around represents (or how much of a drain you are, if it's negative).
Fear is not strictly perjorative; high fear, along with high trust and high value, could have them seeing you as a very capable but tempermental leader.
Right now the only noticable effect of these values is on your persuade/lie chances, and how quickly they'll get fed up with you demanding items. But they'll be used for determining follower status, sharing equipment, taking orders, etc.
I personally would love to see a bit more compex interaction system, with scenarios like:
You throw the stick in the zombies legs and he falls
You chop off the zombies arm
You connect the rope to armless zombies neck (as lasso throw or in melee range)
You light the gas-canister-rocket (moves in a semi-straight+randrom line and explodes at a randrom points with high chance when it hits something)
Basic sling traps would be great, too.
I donīt know, but zombies are not really affected by the amount of damage they took, or? A z with 1 hp left will be just as effective as a "healthy" one?
Sorry, but not going to happen. It works well for DF, but it's definitely not something I want in Cataclysm. Simple, classic roguelike combat here.