Characters don't need experience, levels or stat raising. When a character is created, the attributes he or she chooses should be set in stone. I hate experience, I've always hated experience. It means nothing. Experiencing a bunch of events should effect your personality, your character, your desires, you reasons for doing things - all of which are roleplaying attributes, not stats.
Levels are likewise.
Every player in an RPG should be relatively the same. Since everyone will choose different levels for their basic attributes the variety in the players will already exist. Some people are smarter and others are stronger. The player chooses this at character creation. Players should also have to choose strengths and weaknesses.