more enemies, more damage dealt to you, less damage you give to them, and there are more of the taken then on hard or normal. or maybe it just feels that way on nightmare :shrug:
then I probablly wont play that mode. I prefer metro 2033s hardcoe mode, you do more damage, but so do they. You also pick up less ammo and filters, so in this case it could have been ammo and batteries.
Making them bullet sponges who can 1 hit kill you is just lame and lazy.