Ever noticed, that enemies and allies in HL2 never showed a reaction, if they were hit by a gunshot (excepting death)?
https://www.youtube.com/watch?v=Aaj7z35VTFM&feature=related
That’s like shooting…somehting that feels no pain…and that’s simply annoying in my eyes!
The zombies in L4D for example have hit animations
and also in HL1, npcs performed an animation, too:
https://www.youtube.com/watch?v=KwJG9zMyClk&feature=related
(the arms of the barneys swing backwards, if you shoot them)
Maybe it is a little bit too late for this, but I would really suggest to implement pain reactions!