- abstract measure of ability beyond the norm;
- concrete measure of training and competence.
Trey raised some questions, which I'll deal with here:
"Why is it more difficult for certain classes or races to gain that heroic spark?"
I take this to refer to differences in advancement rates and level limits for races. Many of the differences depend on the local interpretation; obviously, a GM who dumps level limits, as many do, eliminate the differences in difficulty for races. The various random advancement schemes floating around eliminate differences in advancement for classes.
Since the restrictions and different advancement rates were introduced for balance reasons, you won't find many in-game rationales that will work well, although you could claim that it simply takes more of spark -- more experience -- to do the impossible (magic) than to merely excel at the ordinary (fighting.)
That's the only distinction that made sense to me, so I ditched the standard experience charts and went with only two charts -- well, three, if you include "semi-supernatural" hybrid classes. And I'm turning races into mods of classes, so they use the same rules. The races and classes now gain experience at the same rate, but a 2nd level spell (usable by 2 HD magic-user) is equivalent to a +4 to skill (4 HD Trickster) or +4 to reaction (4 HD Charmer.)
"If two characters do the same heroic deed, why do some get less benefit than others?"
I may need an example of this. I think it refers to experience bonus for high Prime Requisite scores, but I'm not sure.
"Why is it so specifically tied to killing and money acquisition?"
Is it? The killing part has been a matter of constant debate. Many, including myself, go with "defeating a monster" rather than killing -- and I count all sorts of things as a defeat, include bribery. Treasure gets debated as well: many (*not* me) ditch experience for treasure. I consider it an abstract measure of problem-solving that doesn't involve conflict with a monster. I also add experience based on ability scores needed for an action.
And again, those who use random advancement, story awards, or experience per session ditch the "killing and looting" problem entirely. In fact, story awards are pretty popular and are completely inconsistent with the "professional competence" interpretation. Why would a ranger gain competence in hunting and tracking, or a thief gain competence in picking locks, simply because they rescued the princess and prevented the villain from waking the lich-king?
Experience awards seem to me to be the one element of the rules that a GM *must* customize to fit the campaign.