Some form of progression is essential, just because of human nature.
I still like the D6 WEG system of skill points that can be used to improve your character skills, or for an emergency bail-out in a bad roll.
I don't need fixed 5000XP=lvl3, level up every 3-5 sessions, but some form of progression.
how would you feel if you progressed through the story, gaining currency, becoming more renown, and obtaining better gear? ...
But seriously, how important is the leveling mechanic to you as a player?
Is it genre specific in importance?
Can a game get by without a leveling mechanic?