I use my own set of house rules for XP, which depend heavily on loot taken (and sold) for determining XP awards. And the amount of loot needed for that XP varies by the character point value of the PC taking home said loot.
Douglas Cole is a big fan of the phrase "emergent behavior." You know, what emerges from actual play when the PCs and situations meet the rules.
You can see that play out in my DF game.
Vryce is 534 points, these days. He was 509 for a long while, saving for Extra Attack 1 . In theory, that's a 5-session savings. If you explore some new territory (1 xp) and take home sufficient loot for your threshold (4 xp), you get 5 xp.
In actual practice, this took much longer - more than twice as long - for Vryce to save up.
With a loot threshold of $20,000, and a minimum need of $4,000 just to get 2 xp (and 0 xp below that), it's been hard for Vryce to make steady gains.
This also pushed Vryce's player to spare Vryce a bit, and play his other character (which he greatly enjoys running, as well.) Vryce is saved (and saves himself) for big scores and dangerous delves. He doesn't risk himself, his gear, his expendables, etc. on low-reward delves or ones he feels aren't up to his standard of challenge. Why risk death for 0-1 xp, maybe 3 xp with a "huge" $4K haul, when he can save himself for delving past gates, striking at powerful foes, or going after potentially big loot? Combat in GURPS can kill a 500+ point character stone dead with a bad series of rolls; it's foolish to do that for 250 point character level rewards.
This has also encouraged people hovering below a big threshold jump to consider saving for something big - a 10, 15, or 25+ point advantage - to make the jump "worth it." This emphasis on big jumps instead of incremental gains also feels good in this game. It means PCs have more of the "big" advantages, and don't just incrementally increase their power at all times.
So this system is working in a lot of ways I had hoped.
- it pushes an emphasis on loot and exploration, with killing a way to accomplish those things.
- it pushes players to risk their delvers on deeper and deeper delves, or suffer a long, slow slog of improvement.
- it encourages bringing along new PCs, since a 250-349 point PC can get 5 xp pretty routinely.
- it encourages players to use their higher point PCs a bit less often, on "appropriate" delves.
- it slows down growth from purely linear, so characters start to slow down in growth and don't make a steady march towards "everything, all maxed out." You always have to make tradeoff decisions, even at 500+ point power level.
Overall, it's been an interesting experiment and it's had a lot of the positive effects I hoped for, and some I had not. It's worth giving some thought when you design a system of XP (or another rules system.) Does it potentially encourage the kind of things you want to see in the game? I wanted to see an emphasis on loot, secondarily on exploration, and a desire to risk your high-value PCs less and less.