Slowly leveling up. I hate it when you start out, and everything can kill you in like four hits. You should have the same health and take the same damage throughout the game, on a sensible scale.
And then everything gets boring when you reach a certain level because you can kill everything in one hit.
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"All right, brain, I don't like you and you don't like me--so let's just do this, and I'll get back to killing you with beer." ~ Homer Simpson