For instance you can strengthen the limbs of your bow for more damage, increase the clip sizes for your guns by cobbling stuff together and create
napalm arrows for some added enemy misery.
This wouldn't have felt quite so bad if it had been done in a way that made every kill feel like a necessity, but it doesn't: you'll set enemies on fire with
napalm arrows, stick pickaxes in people's heads, pull them off cliffs and shoot them through the head at point - blank range.