Continuing discussion on experience/levels below, another idea...

My main concern with experience and levels is that all players currently gain experience at exactly the same rate (2 jumps per day, 1 experience per jump). I've been considering the following idea:

All items have an integer property called "Karma", which defaults to 0. Whenever a player uses an item, that player will gain/lose the amount of experience indicated by the Karma value of the item.

And now for some possible Karma values:
Mushroom: 1
Feather: 2
Coin: 2
Fire Flower: -1
Ghost: 0
Green Shell: -1
Red Shell: -2
Lightning Bolt: -5

Under this system players would be rewarded or penalized for taking "good" or "evil" actions.

Does anyone else have any other ideas on how we could make experience actually accumulate differently for each player?