A little videogame theory, from someone who ponders this sort of business more than is good for him:
We know what XP is in meta-game terms. It’s supposed to represent the acquisition of knowledge. So, the thinking goes, after you’ve killed a hundred orcs, killing the 101st shouldn’t teach you anything new. I could argue this point and we could get into all sorts of simulationist arguments about what would produce the greatest fidelity to real-world behavior, but the truth is that in gameplay terms XP is really a reward for risk and effort.
Most games have you earning XP on an upward curve. As you proceed, the monsters are worth more XP as well, but the rewards don’t quite keep pace with the XP needed between levels. So, maybe you only need to kill ten monsters to go from level 1 to 2, but you’ll need several dozen to go from 5 to 6 and a hundred to go from level 19 to 20. (This is all assuming the monsters you’re fighting are the same level you are.)
Some games feel the need to impose a certain degree of risk on the player. You get penalized for fighting stuff below your level. You’re level 10 and you’re fighting a level 1 rat. That rat would be worth 10XP to a level 1 player (a pittance to you, a level 10) but if you kill the thing you get zero. Most games make this restriction pretty tight, so that even a monster slightly below you in level is worth far less than it was when you were “supposed” to be fighting it.
In your standard RPG / leveling kind of game, the player should always be compensated for risk or effort. The only time a monster should be worth zero XP is if the player can kill the thing in a single hit, without breaking stride. Anything more than that, and the player deserves a reward for putting the beast down.
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