Difficulty Level

The difficulty level affects a number of things. For example, pirate crossbowmen will fire their arrows predictively, toward the position you're currently headed to, rather than directly at you. There are also differences in numbers like enemy HP, and the experience they grant (higher difficulty levels make enemies give less exp because their spawn numbers are greater, so if you can clear quickly you'll gain levels at roughly the same rate).

For easy, normal, hard, and serious respectively

Enemy health: 66%, 100%, 125%, 150%

Damage taken from enemies: 66%, 100%, 125%, 200%

Damage taken from traps: 50%, 100%, 125%, 150%

EXP scale: 100%, 100%, 75%, 75%

Revive time (seconds): 1, 3, 3, 6

^ Should be noted that EXP is also affected by your tier vs that of the enemy, and it will thus be lowered if facing enemies of a lower tier. There isn't currently anything that indicates this, aside from higher-tier enemies having darker HP bars.

The EXP difference is as follows:

Apprentice: 100% (all)

Adept: 50% (vs apprentice), 100% (vs adept/expert/master)

Expert: 20% (vs apprentice), 50% (vs adept), 100% (vs expert/master)

Master: 10% (vs apprentice), 20% (vs adept), 50% (vs expert), 100% (vs master)