The flashlight dies over time, and you need gasoline to power the generators in "Into the Abyss". Allegedly, Slender Man and the hoodied creature chasing you in the mine are also more aggressive. I am not aware of any other differences, but I've not played through it myself; I've just seen the 4Harmes LP:
Your flashlight can run out of batteries during any level (It refills at the beginning of the new level).
Your stamina drains faster
Changes by level:
Nothing in level 1.
In level 2, Slender Man is more aggressive, but the number of pages is the same.
In level 3, you have to find 6 fuel containers before you can activate the 6 generators.
Nothing in level 4.
In level 5, Slender Man's tentacles move faster, more paths are blocked off by fire, and fire kills you faster. --- "Playing the game stops at hell, buying your way threw the game is what inferno is all about." -Northern_Rose on Diablo III