Artificial difficulty is, as several others have said, when player skill is minimalized or removed as a factor of success or failure, and is instead based heavily on gear checks (hp, dps, etc).

Memorization, adaptability, reflex, rotations are all measures of a player's skill and thus things that create the difficulty of an encounter. Having mechanics based on having X amount of HP or being able to do an amount of DPS that is unachievable without certain amounts of gearing is what adds artificial difficulty to an encounter, but this game is designed to have tiered gear and tiered raids.