Instant Gratification
One feature I've always liked about Chaosium's system for Call of Cthulhu (based off of their Basic Role Playing) is the opportunity for character improvement after each adventure. Unlike level based games like D&D, or even point buy games like White Wolf's World of Darkness, your character has a chance to advance each of their skills after each adventure; the only caveat being that you have to use the skill successfully at least once during the adventure. Talk about instant gratification! You could easily do this with D&D, just do away with purchasing skill ranks at each level and allow the player to do a skill check after each adventure with every skill that was used successfully within each adventure. If they succeed, allow them to roll 1d4 and add that many ranks to that skill.
