- Doom will be going back to DLLs, and will be using C++ (with the exception of the rendering code) instead of C
- Carmack feels the visual 'wow factor' of the new Doom will be a greater jump than it was from the recent jumps between Quake/Quake2/Quake3. (Lots of neeto new effects), but it will also focus on actual gameplay instead of just that 'wow factor'
- Single player is the absolute focus, and Carmack feels it will be their best single player experience to date.
- The game and editor/tools are now combined. You'll be able to simply type doom.exe -editor for example (Or maybe even an in-game toggle)
- Spending more time on the sound code / game code with the new programmers on the team.
- There probably wont be much discussion on the gameplay specifics since what made Doom so big is always a subject of arguments. (No talking about the game itself)
- All of the Doom code is being written from scratch. Rendering etc.
- A port to XBox is likely, Linux is likely (although it will take some work) and Mac will be OSX only.

6pm update: I just realized I left out one important detail. Carmack mentioned off-handedly that there will be a test release of the new Doom.