In classic D&D, XP is rewarded for killing critters and picking their pockets clean. As you increase in experience, you fight tougher monsters (more XP) who give you more treasure (more XP). So the game is an ever increasing ladder of tougher creatures and bigger hordes of treasure as the characters gain XP. So what does this tell the player about kind of game they can expect to play? What does this tell the referee about the kind of game they should be creating? Is there room in such a system for being anything but murder hobos? Conceptually yes, on terms of rewarding player efforts in the game for doing so? Not so much.
When I was designing Heroes & Other Worlds, the question of awarding XP was troubling. My own prejudices against treasure being a dual reward (better/more equipment & XP reward) meant no XP would be awarded for treasure.
Second, while I agree that tougher foes means better loot, that is also in itself the reward for the risk. At one point in design, the XP of a foe was equal to the creatures ST. Yet, that did not last long in play test. It worked, but it did not fix the original limitations I saw in D&D. XP awards for the death of foes only leaves the range and options for players to interact/defeat foes rather one dimensional.
Last, being a game with a range of skills, I needed to insure every skill from sword fighting, to spell casting to tracking or acting had an equal XP weight in the game and the equal opportunity of being rewarded. In other words, when all options or actions provide the same potential XP reward, then the adventure design potential becomes wide open for a referee, and the possibilities for players to act also becomes unlimited.
In Heroes & Other Worlds, you roll 3d6 for a test. The sum of the roll is compared to the total of the Hero's Attribute+Skill level. Roll equal to or lower...and you pass the test. Each test passed earns you 3XP. That's it clean and simple. Any test from casting a spell to freeze a Yeti, using a bow to slay a dragon, or juggling fruit for coins in the market, all provide the same XP reward. This is the basic reward system. There is an advanced option the changes the reward amount for a success, but the system for rewarding XP is the same.
My goal as a designer was to reward action, and XP is rewarded equally for any successful action a Hero may choose to make. There is no XP weight in the game towards combat, non-combat, or spell use. The system rewards successful actions of any kind equally. With this flexibility it is my hope Referees will be free to design adventures and situation open to multiple resolutions, and knowing this, Players will also feel free to pursue any option they can think of.