I liked the way health worked for the marine in AvP. You had 3 regenerating bars, that once debpleted needed a health kit to refill. A light injury would take half a bar, a normal one a full bar, and headshots and the like 2+ bars.
And the bad thing about how ME2 dealth with enemy health on higher difficulty levels is some of skill/powers didn't fit with the system. I started off on Hardcore then Insanity after that, and remember checking Normal (or whatever fancy term they used) and thinking "Wait, these guys only have a single HP bar, wth?"