Mental Encumbrance
I like encumbrance systems. Deciding whether to bring this or that equipment, or debating whether or not to drop your sword so you can carry an extra bit of treasure on your way out of the dungeon is a very interesting decision and makes the game all the richer for it. Without an encumbrance system, there's no penalty for carrying loads of useless items, so these decisions never have to be made, and there's no reward for a clever player. It is also realistic- you just can't carry everything, realistically. A penalty for excessive physical burdens is good. Why don't we have one for mental burdens? Just as there's a limit to how much you can carry on your back, realistically, there should be a limit to how much you can keep in your head. It's very rare you see an RPG give characters any penalty for knowing lots of lore, for speaking many tongues, or for knowing a whole library worth of spells in their head. Typically, this penalty for being physically encumbered