One reason mob grinding might seem boring to some players is that one can do it without putting oneself in danger. The typical mob grinder experience is sitting behind slabs swinging at the mobs' feet for a few minutes & you have hours worth of XP. It's far too efficient. Something messier is needed.
A configurable timer can be placed on a mob on hitting a player, & if this counter isn't present on that mob, they drop no XP.
Obviously not all mobs can hit the player in a way that allows them to be hit back, such as creepers, so a configurable list of exceptions to the counter rule should be available too.
This'd be in the following categories for the following reasons:
Server Utility: Balances economies, PvP, PvE, & a lot of other things too.
Adventure/RPG: Dramatic change to typical leveling routine
Magic: Makes getting enchantments much more gratifying, since you gotta work more for it.
& maybe Cosmetic: Mob grinding will be a more sensually stimulating experience.