Hilarious in Hindsight: This fic has Luffy imbuing his attacks with Conqueror's Haki years before it was confirmed possible in canon. Not only that, the effects are far more powerful in canon than they are in this fic.
Espers and their Focus ability, which makes every attack hit.
Darkfolk and their Boom spell, which deals damage equal to your current mana multiplied by your power. Upgrade your weapon (which boosts your mana pool) and power, and you'll soon be capable of dealing a One-Hit Kill to every enemy in the game, including the Final Boss (which only has 777 HP). The only weakness to this is that if you miss, you'll have to recharge your mana before trying again, but this can easily be remedied by offering lots and lots of gold to Spediphis' church to boost your dexterity. Your armor will fall into the negatives, but that won't be a problem since you'll be annihilating every enemy before they can even move.
When you remain in an area for too long, the enemies stop becoming stronger and you'll end up fighting the same enemies over and over again. This usually happens when your stats are so high that killing these enemies is no big deal. These enemies include Dungeon Wanderers, Elite Orcs, Ent Ancients, and Discreet Bears.
Parodied with the Goddamn Friggin' Bats. The player character finds them infuriating to deal with, but gameplay-wise they are relatively easy to beat.
Good Bad Bugs: Quitting and reloading a saved game causes the game to become somewhat unstable. This can cause your character to have infinite lives, potions, multitools, attack modifiers...