Immersion in the game world, something that sucks you in and makes it feel like a real believable world.
A lot of developers/designers seem to mistake what difficulty actually is these days. Difficulty shouldn't mean that your forced to hide behind cover for 10 mins, stick your head up and take about 3 shots before every single enemy's fire is directed at you, hide behind cover again while your health regenerates, rinse an repeat. In the case of a fps anyway. You need to find a balance of challenging the player, while still providing fun and engaging gameplay.
A lot of developers/designers seem to mistake what difficulty actually is these days. Difficulty shouldn't mean that your forced to hide behind cover for 10 mins, stick your head up and take about 3 shots before every single enemy's fire is directed at you, hide behind cover again while your health regenerates, rinse an repeat. In the case of a fps anyway. You need to find a balance of challenging the player, while still providing fun and engaging gameplay.