I started with the usual system for damage with some minor tweaking due to lack of levels; subtract damage from your hit points, unconscious at x and dead at y. I later added a fatigue system to deal with spell failure and special abilities (kinda like mana but not restricted to magic). I then added on a wound system. Hits that did a certain amount of damage caused a wound point, when you received a certain number of wound points you got penalties to your rolls. I decided that this was way too much number crunching and slimmed things down.
I got rid of hit points because they were too abstract for my taste and didn't do anything that couldn't be done better/easier elsewhere. Any hit that does damage gives one point of fatigue, as does using special abilities and not sleeping properly. Fatigue represents physical and mental wear, when you receive a certain number of fatigue points you get penalties on your rolls. I took the penalties off of wound and made it so that you died when you accumulated a certain number of wound points. This is still a little too abstract and clunky for my taste.
I am changing fatigue so that every point of damage gives a point of fatigue instead of every successful hit, however I have not settled on a satisfying method for wound. There are some things I am considering. One is including hit locations. There would be 5 hit locations(head, torso, right and left arm, right and left leg) and each location can only take a certain number of wound before being irreparably damaged. Another idea is having escalating wounds. When you receive a wound, you have a certain number of rounds to tend to it or it becomes a serious wound and then a mortal one.