that system is already drastically altered
. seems like the amount of exp gained is based on level difference on a semi-linear scale (to a certain point) so that killing a monster higher than your level can be worth it:
Equal Level: 100% exp
Monster 1 level higher: 101%
Monster 2 levels higher: 102%
Monster 3 levels higher: 103%
Monster 4 levels higher: 104%
Monster 5 levels higher: 115%
Monster 6 levels higher: 116%
Monster 7 levels higher: 117%
Monster 8 levels higher: 118%
they already fixed the stupid 100% damage reduction against people of lower level thing to a certain degree (no more clowns easily tanking maya). seems like damage is reduced (as well as def and hit) slowly as the enemy you fight increases in level. so fighting a monster 5 you might feel the effects of reduced hit, damage, and def but is always possible to kill (you can attack a wider range of monsters than previous versions of beta testing).
its also speculated that as your level gets higher, these effects are reduced such that if youre around 99 or higher you dont need to worry about these penalties as much. makes sense to me since people spend more time in the higher levels than the lower levels (dont wanna be a level 120 trans unable to touch a level 140 mob...)