These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
Game Breaker: Many players have noticed just how powerful the Engineer's skills of Super Iron Man and Black Explosive are when used on a Mimic.For further explanation... Black Explosive is an attack skill which does fire damage, has a small cast time and can hit multiple times under certain conditions; Super Iron Man is a buff that increases stats for a short time and doubles the damage of two of the engineer's skills, one of the two being Black Explosive. While on Engineer proper, the damage this skill does is mitigated by the fact it requires a gun equipped, and guns have a low base magic attack. Cue mimic, which can use any armor and any weapon. Because of how the mimic's mechanics work, it's entirely possible to cast skills that require a ranged weapon while having the skill run off your melee weapon's stats... where the staff, highest base magic attack of all weapons in the game, is considered a melee weapon.
That One Attack: Unremovable damage-over-time attacks which stack and damage reflection buffs/debuffs. Made more annoying/challenging by the fact that most player interrupts all trigger the boss'sMercy Invincibility timer and the bosses can spam the attacks.
They Changed It, Now It Sucks / Broken Base: The Bard skill "Burning Metal" had been changed in a patch in Aeria Games' version, given a 2-second cast time from previously being instant-cast. As this change effectively changed its kiting abilities as well as its use in normal parties, many people were unhappy with this change, while just as many were happy about it since then other people couldn't 'exploit' it for said kiting abilities.