I've read the preview now as well and have a few more details to add, but i haven't followed all the news we got so there may be stuff that was already mentioned.
- day and night cycle has an impact to certain monsters. Some will have different stats depending on the time you fight them
- there will be secret locations that are only accessible during certain times of day or even during certain weather conditions (a magical door only opening during night or while it rains)
- apparently, Geralt can swim (they mention him taking a boat, because swimming in the ice cold water would freeze him to death)
- improved combat system: a buttonpress is now a single attack, instead of the combo stuff from Witcher 2. The camera will also zoom out during encounters.
- enemies can yield
- the igni sign sets the surroundings on fire
- crafting recipes as in Witcher 2, but it seems you'll be able to use the same crafting recipe with different materials (using dragon scales instead of leather to craft an armor).
- CD Project is currently adding DX 11 effects to the engine and the game will look a lot better when they're done with it.