I may need to double check the specific rules, however, it's mostly just for how it handles negative HP and healing effects (for example, I may, or may not, institute a rule along the effects of getting hit forces an immeadiate save, or that it adds a penalty to future saves, or something similar, just to make it so that degree of damage matters).
However, the basic mechanic is a three strikes system running off of your saving throw, which means that a bleeding human often lasts longer than a bleeding anything else. Also, rolling a 20 means you stop bleeding (but don't get up) And yeah, it was pretty similar in 3.X, except that negative HP mattered, and the mechanic was that you'd roll a d10 and you'd lose 1 HP each time you didn't roll a 10. Of course, if you did roll a 10, you were home free until you got hit again.