Going to start posting some ideas I have.

[kato13]

Absolutely no levels.

My system has the following high points:

A semi bell curve percentage system for task resolution.

Think of it as a D19 but based on percentile.
You only get a 1 or 19 (really really bad or really really good) 1% of the time for each. While a 9, 10 or 11 (pretty average) occur a total 28% of the time. To me this is much more realistic than straight D20 with everything having a 5% chance.

Skill acquisition cost increases at higher skill levels.

A skill level of one costs 1 point. But a skill level of 4 costs 10 points in total (4+3+2+1). To move from that 4 to a 5 costs 5 but to move from 5 to 7 costs 13 (6+7). Overall in my system this increases the number of low level skills and severely reduces high level skills. Just like in real like you can either be a jack of all trades or a master of one.

Skill learning can be enhanced or degraded by aptitude.

Attributes go from 1-10 and are connected to skills. If you have a 5 or 6 attribute it will not effect your skill learning. If you have a 1 attribute the connected skill is 3 times as hard to lean. If you have a 10 it is only 1/3 as expensive.

This still allows for a "natural" to have a very high skill level.

Very little enhancement of player durability
No massive hit point increases. You can enhance your ability to take damage slightly via exercise, training and conditioning. Drugs can also give a temporary boost as can adrenalin. But it is difficult to count on that.

I am still trying to tweak the rules to allow for those amazing wartime stories you hear about someone having 6 ribs sliced open yet still carries their buddy 3 miles to safety. It would require some sort of amazing role, but it should still be a possibility.