Chaosium's Basic-Role-Playing system--which powers Call of Cthulhu (and other games) is one of the most elegant game designs ever. Love it.
A nifty feature is the skill system--if you're not familiar, it works like this:
-Every time you use a skill, you mark it on your sheet.
-At the end of the adventure (or chapter, or whenever the DM says) you roll on all these skills that you've used lately (trying to fail, actually, but that's not important here).
-Rolling the right numbers means you get to add points to that skill.
I feel like there's a way to combine the "Hey, use your brain, not your character sheet" ethos with a skill system in D&D by doing something similar to BRP, but more open-ended.
At the end of a session or whenever x.p. is given, a player can choose some ONE thing s/he did during the period x.p.'s being awarded for and spend experience points (I'm thinking about a fifth of what it'd take to get to the next level, but I'm not sold) to get better at it.
The details are still vague, but the outline I'm thinking is:
-The main idea would be to get better at things not associated with your class--i.e. if a warrior wants to get better at identifying unknown languages, f'rinstance.
-Combat skills are ok to advance, but I imagine the things would be stuff that normally you'd be at a minus to do. Like if you want to learn to fight with two weapons at once, you have to start out fighting with two weapons at once with a minus on both hands (the AD&D recommends -2 for one hand and -4 for the other, I believe, but I am too lazy to get up and check), then slowly buy your way out of the penalty, one adventure at a time.
-The skills could be goofy. Like if you sold a rust monster to someone that day you could get a bonus to selling monsters in the future.
-DM gets final call on what can be considered a skill. Duh.
-As usual, the player would have to keep track of all this stuff. If you have a bonus, it's up to you to remind the DM that you think it applies in a given situation. Players are usually ok with greater levels of complexity if: A) It comes after they've generated the character and B) It makes them more powerful.
-The main question is the cost, I suppose. I figure each PC in most versions of the game gets 2-5 new "things" per level (hit points, combat table bonus, save bonus, a special ability or two--or spells). Essentially the idea here is you're buying a "thing" not germane to your normal class advancement, so that's where I got the one-fifth figure. I think the only way to be sure is to try it and see if anybody bites.