Looks to me mainly higher res textures and bug fixes.
Bug fixes, yes, higher res textures, not really, if anything, they took the updated textures and materials from Episode Two
They've also fixed up the draw distance, fog, added light rays to certain places, fixed missing geometry in some areas, added HDR, changed some particle effects, improved the blocky lighting and lack of shadows, and changed the commentary mode to feature some well known Youtubers from Did You Know Gaming and Ross Scott