Link to article on Toms Hardware
Based on this article it sounds like ID Software spent quite some time on tweaking original Doom 3 game. They were even changing some original levels and made the lighting setups brighter. Generally applying their current perspective on how they think the game should look and play. Plus networks code from Rage to improve multiplayer. This is on top of new levels Lost Mission, PS3/Xbox support, 3d glasses support and higher resolution textures (and all original Dooms). The release is now in around 4 weeks - 16.10. in US and 19.10. in Europe.
Here is the full interview with Tim Willits