Level requirement for equipping items.
You find something that looks good, but no, must be level 17 to wield stick of slightly better than current stick.
This can make a rewarding exploration design feel unrewarding just by itself.
You manage to defeat an enemy 5 levels higher than you, find an epic item in a chest behind him, celebrate your victory... and then realize you can't equip the item until 5 levels later.
Ok.
So managing to defeat this enemy against all odds to get my hands on this epic item did... exactly nothing. I can't use the thing until I leveled up several times, and by that point the almost impossible enemy I just defeated would have been easily manageable.
Meh.