Game breaking mechanics. For instance if you kill a late game boss Malekith, an NPC that holds all the late game spells and abilities dies. If you didn't visit him before hand you essentially ruin your save file. (mines 100 hours leading to now)
Punishing mechanics like this replace actual mechanical difficulty, since you can level to the point you one shot bosses. Games beautiful and clearly been worked hard on, but it's poorly designed.