In the original edition of D&D, characters received experience points both for defeating monsters and for the treasure they gained doing so. For some reason this was not well-liked -- especially the experience for treasure. Later editions started moving away from this. First, experience for treasure was dropped. Then experience for defeating monsters slowly transformed into experience only for monsters slain -- at least in most campaigns. Additional sources of experience were added along the way (like a few hundred xp for good roleplaying), but most of these additional sources did not provide nearly as much experience as treasure did, so the emphasis of the game slowly shifted from exploration and treasuring hunting to killing things.
Last night, I wondered what might have happened if experience awards had shifted the other way. What would a game be like that only gave experience for treasure collected?
After thinking about this a while, I decided it might make for a far more interesting game than one might expect, especially if the campaign was set up around this idea. Let's establish some interesting ground rules.
1) Treasure is converted to experience by spending it on training. If you need 10,000 XP to go from level 5 to 6, you get the xp by spending money on training. When you've managed to gather and spend 10,000gp on training, you go up a level. Unlike in AD&D 1e, training need not involve finding a teacher and taking time out from the campaign to train, it is just something you can do in spare time.
2) Treasure that you choose to spend on other things (like upkeep, healing potions, equipment, taxes, etc.) does not earn xp.
3) NPC rulers often give bounties on certain monsters killed (a bounty about equal to the xp the monster would bring for killing it). However, you usually have to bring some type of proof that you actually killed the monster (a head, a tail, etc. depending on the monster) to the NPC ruler. Which monsters have a bounty and how much can change with location and time in the campaign.
A campaign using this experience system would put more emphasis on finding treasure or otherwise making money than it would on killing things. Given that bounties are only given for certain monsters, there would be less reason for players to have their characters want to kill everything that moves. Having to bring proof of kills to get bounty money would make some spells (like disintegrate) and some tactics (e.g. pushing monsters into lava) counterproductive with bounty monsters. Finally, by requiring buying training with gold to convert it into xp, there is a somewhat logical campaign world reason for treating treasure as xp.
As you can tell from this short writeup, I have not completely thought this idea through -- let alone tested it in play. I'm interested in hearing what others think of it before I do much more with it.