does aim even really matter in L4D? like, I'm sure there's a difference between head and body shots. but does it differentiate between torso/waist/leg/etc.? in bf3, for example. I know it does. like with a crossbow, if you hit them in the head or chest with a regular bolt, it's a one-hit kill. but anywhere else, it takes two (which is a REALLY bad idea bc of the super-long reload). so you really have to focus on getting chest, because trying to hit the head is unreliable unless they're standing still. and I think that's really interesting, the whole re-learning thing. but otherwise, using firearms, my aim usually starts at the chest, and naturally drifts up to the head from recoil, but also on purpose, bc it'd be easy to keep it on the chest if I wanted to :P
The damage barely differentiates from the chest and waist, but it definitely does change from the arms to the legs. It should take about three or four shots to the chest to kill a Smoker with a hunting rifle, whereas it would take one to the head to kill it, about 7 shots to the legs to kill it, or 10 shots to the arms to kill it. The timings really matter though, as soon as they stand still to grab you is when you aim for the head and fire. The Chargers are really hard to kill with a hunting rifle unless you get the two headshots in on it, and in 2v2 Confogl aim can also mean life or death when a Jockey or Hunter is on your teammate.