I played about 1 1/2 hours last night on a new game.
I've only gotten to the parts around Ardea, Reddock, and Cape Dun so far. Haven't noticed any plot or quest difference up till now but I think there's snippets of new dialogue already. New characters as well (for instance - there are fishermen at the fishing village where Lester is now).
Also
MUCH less stuttering. It's not completely stutter free, but it's more on a normal open RPG scale like Oblivion (about a seconds worth of stutter when you go towards a new area and pretty smooth after that). A good example is the entrance to the cave at Reddock. My PC would stutter whenever I entered it for the first time each game before 1.7, but after the patch there was none at all.
More detailed graphics (a Very high detail option). I was able to max out the game with my 8800 Ultra, so any newer card should make it even smoother.
An option to re-balance the game when you start and improved combat. Again - I haven't been deep enough to notice many differences, but I did do a few arena fights and the combat seems better to me. It's more on par with combat from the earlier Gothics without having to press the ctrl key to attack.
So far - This is what the game should have been. It would easily have been considered an equal to Oblivion and surely a better game by many people.
If you're lamenting the lack of PC only games (especially RPG) and you never tried Gothic 3, nows the time to give it a shot.
The CD check removal from 1.6 intact.