Choosing a rubric for awarding XP is key, as it shapes how the game is played more than any other aspect.
As I see it, XP can be used as a reward in several different ways:
- XP can be rewarded for completing goals (like Belief Artha in BW, or resolving a Mission in Reign)
- XP may be awarded for overcoming obstacles on the path to a goal (standard D&D)
- XP can be gained as a mechanical interpretation of, or reward for, character development (such as the classic D&D treasure spending "party binge" rules)
- XP can be a compensation for a character's troublesome learning experiences (such as Problems in Reign, or Trait Artha in BW)
Each kind of reward mechanism encourages its own kind of play, and forces the system to adapt to its underlying assumptions. For example, the standard D&D XP system rewards killing monsters & generally winning fights with increased abilities, mostly aimed at winning fights, but also at all sorts of other stuff, from persuading people to knowledge of ancient history to skill at making horseshoes!
By the way, one of my base assumptions here is that a D&D-like game has to award XP, spent to improve your character's abilities by gaining levels, rather than having any other kind of rewards, such as BW's Artha resource, or Reign's continual bonuses.
One of my biggest problems with modern interpretations of D&D is that they have made XP & levels completely separate from the reward system; they had to, thanks to the rapid increase in levels and sheer disparity in both depth and breadth of power that characters obtained with leveling. To undo that, characters of differing levels have to be able to meaningfully interact, which means that levels have to either result in less of a power boost, or that they only confer greater ability on a very narrow tract of the gamespace.
One of my secret desires is to have an adventure game where the possession of an artifact or secret magic is just as (if not more) important power-wise than a character's innate levels. Once again modern D&D has issues, as the 3e/4e systems evaluate characters' abilities expecting them to have a certain prescribed set of treasures: all hail the almighty leveling ladder.
This raises an important point: traditional D&D has not one, but two key rewards: Experience and Treasure.
(It might have a third as well, Status, but i'll discuss that later, since gaining power over the world, rather than power of self, makes for a natural "second tier" of D&D play. It seems likely that you'd have three tiers, completely separate from modern D&D's idea of the "rise to epicness", in my reconstructed S&S game: 1) Heroes, 2) Kings, and 3) Planeswalkers. However, i'm not sure how to keep them from becoming intermingled...)