I don't like being able to be killed with no response time... I'm not a fan of over realism (for example in an FPS I would prefer to be able to carry many weapons rather than being limited to say three and having to swap with other weapons)
As such, with sniper rifles, I think a headshot should constitute maybe 80-90% health loss. The only things that should be 1 shot kills are a direct close range shotgun blast to the head (ie the enemy is taking up half of your screen range) and very close range explosives (ie within a 30cm locus of your character, which basically would mean a direct hit with a rocket/grenade launcher)
Oh, and health packs > regenerating health.... though ultimately I think a combination would be best. e.g. your health regenerates, but only to a point. If you have 90% + health it will regenerate fully, 70-90% will regenerate 10%, 50-70% will regenerate 15% and so on.... but otherwise you need packs to get back to full.
