Theme music for this post is the Apocalypse Now remix from my signature: https://www.youtube.com/watch?v=vrYvcFDNmBM
So I was considering the un-realism of how in most role playing games the more you engage in deadly combat, the more EXP you get, and the more powerful your character becomes.
In reality, though, excessive exposure to real combat makes you weaker instead of stronger. For example, the current military conflicts and Afghanistan have now been going on for longer than the entirety of the Vietnam War and you many military personnel with more multiple tours than previously in recent history. One impact of this which I have read about is more incidence of traumatic brain injury and resultant long term health and cognitive issues from repeated exposure to explosions. Also, any athlete knows that getting injured, especially repeatedly, basically hurts your performance in the long run.
One of my favorite topics is the Vietnam War and it's sort of a truism that the US military was at a peak of training and technological prowess (including the innovative airmobile concept, arguments about whether traditional armor units would have been more effective notwithstanding) before Vietnam, and that the military was actually greatly weakened as an institution by long term participation in the Vietnam War. You had the emergence of drug problems, racial tensions, pervasive contempt for officers, and lots of personnel getting exposed to agent orange, not to mention hepatitis C apparently, if you really do your research. One author put it somewhat poetically that the shining sword of the US military became tarnished in the jungle mire or something like that.
So what could be better in terms of a realistic feel for gaining fighting prowess through experience in role playing games? How about:
1.) Physical combat skills may be increased through time and money spent on training with factors to influence it such as the quality of the trainer(s) and the physical fitness of the trainee. Therefore a character who wants to be really good at swordsmanship, for example, must spend all his time and money training swordsmanship, which makes a lot of sense. There should basically be a peak level of skill you can maintain with full time training that degrades slowly if you don't continue the training. Thus obtaining a certain physical skill at a certain level isn't a permanent milestone you simply pass, but rather something that requires a sustained input.
2.) Have statistical penalties apply to actions taking under stressful combat conditions. They wouldn't apply for sparring or demonstrations, but there would be a minor penalty for intense competition (like participating in a boxing match in front of spectators for a big prize), a moderate penalty for a real fight (some thugs jump your friend who was walking 20 feet ahead of you and start beating the crap out of him; if you pull out your handgun and try to shoot one of the thugs you get a moderate penalty for using your skills in a combat situation), and a major penalty for being in a real fight under fire (the thugs jump you and are beating the tar out of you so when you try to fight back with your boxing skill you have a major penalty).
This would also allow for the basis of a suppression fire mechanic because you have a bigger penalty when you're being attacked versus when no one is directly attacking you but you're still operating under combat conditions.
3.) You get points of some kind for being in combat (i.e. operating under these penalties) that can be cashed in to reduce the combat penalties to some extent. So this represents stress innoculation or becoming seasoned and in a way is kind of like your traditional "EXP" where you get better at combat from being in combat.
4.) But to counter balance this, you also accrue mental trauma points for being in combat. Each time you're badly injured, have a friend die, or each time you kill someone, you get a certain number of points, that have a chance of producing mental disorders from a table. So you don't really cleanly come out ahead from having buckets of combat experience as you can potentially get a significant setback on the mental disorder table.
5.) Finally, age and injuries reduce your physical attributes over time, so as you get older and more beat up, your combat skills themselves slowly decline even with max inputs.
6.) For players who just want to indulge in combat, there could be a "sociopath" player character quality available. "Sociopath" would eliminate mental trauma for anything but severe personal injury, but would also apply severe penalties for any social skill checks, negotiations, diplomacy rolls, etc. due to non-existence of empathy. So the player who pumps up these skills to compensate would be considered a charming manipulative sociopath whereas the one who focuses on combat and doesn't care would come off as a dangerous, crazy biker or something like that.
I actually think it would end up being pretty hilarious unintentional social commentary if the entire party decided to all play sociopaths, and you had a campaign about a squad of socially disconnected sociopaths running around in a world where no one else they run into is a sociopath.