How many times have I not played an RPG and just needed a couple of more points to level up. XP sharing to the rescue! I think it would be really cool to be able to treat XP as any other resource in a game - this mechanic should of course be balanced and made into something more action filled than just being able to donate a couple of points to another player. Perhaps it's vital for team tactics to boost one of the characters to a super high level to handle an enemy, or perhaps you actually have to use XP as a resource to buy stuff - so when you're buying new equipment you'll have to make sure it doesn't make you less power overall.
Perhaps you even could steal experience from enemies. You get a certain amount of XP from killing a certain enemy, but by stealing XP from it you can both make it weaker and gain more XP at the same time.