General features:
Whole new lighting engine, also supports 1 extra hardware light
Many locations have additional lights and / or tweaked lighting
Anamorphic lens flare for the flashlight
New sleeping and game over music
New title screen and game over screen
2 additional locations
Hundreds of small bug fixes, balance tweaks and improvements
Automatic monster-facing on gun-draw
A few small easter eggs
New short sidequest
Ending collection
The Ritual
Specific to New Game+:
Around 6 new locations
2 completely new endings
3 new songs
Over 20 new items
2 new sidequests
New enemy
New dialogue for all characters in the game
Many other easter eggs that are better left unspoilt