How does this differ from Hard Mode?
You're more vulnerable, enemies are tougher, every resource is more precious.

Why did you guys decide to make this?
When we designed the original three difficulty modes of BioShock, we tried to define a spectrum encompasses as much of the shooter audience as we could, balanced as optimally as possible. However, we know there are gamers out there - ourselves included! - who are after a greater challenge than is provided by Hard difficulty. Besides that, a lot of Bioshock's game mechanics become more interesting when the player is under pressure to use their abilities in a creative and efficient manner. So the impetus came just as much from the desire to highlight a less obvious dimension to the game's mechanics, as to cater to the upper echelons of player skill.

How will my play style change when I decide to play on this mode?
You won't be blazing through spaces. You won't be flush with resources. Every fight is a decisive, life-or-death struggle.