Of course Save/Load!!
I so like to save, go for a walk, load, do 2 steps and 1 shot, save, go to sleep, load, and so...
But mostly i love to save in Nightmare!(tm). Ooh, it's true never ending pleasure for me!!
Just kidding, i want to see moment, when zombies will be so intelligent, that if you leave them for a moment, they will begin to build their zombie society, with little houses and candy trees. And they will promenade with zombie girls, have home imp, and they will grow little Lost Souls, as their zombie childs.
Just kidding. I'm playing generally from 3 a.m. to 7 a.m., so i don't have urgency at this time to leave computer and go somewhere, so i'm fully for new brains for our dumb foes.