I'd like to see a change in the damage rating of firearms. Condition of a firearm doesn't affect it's damage. Once the trigger is pulled, the pin hits the primer, and the bullet is air born, it's going to do the same amount of damage regardless of what the rest of the firearm looks like. I like that the weapons get damaged and have to be repaired, I just wish it didn't affect damage. Maybe with the energy weapons, but it doesn't apply to projectile weapons. A bullet fired from a brand new gun and an old rusted out pile of crap, do exactly the same damage.
Or maybe instead of affecting damage, it affects the weapon spread, with bullets pissing about all over the place. Or it jams all the time and has a risk of exploding (like old guns do in Far Cry 2 - you learn pretty quickly to use high quality weapons except in desperate situations).
It would also be good if the condition affected the appearance. A chinese assault rifle kinda dissapointingly looks exactly the same even when at 100% condition in the Operation Anchorage simulation (when surely it had only been manufactured say 6 months previously).