From what I've seen of the scans it seems a lot like the Pokemon Colosseum games in the graphical sense where you can see your overworld from a distance and the fact that everything is in a lot more 3D. It's far more graphically advanced for a start. But for other things I expect to see well, I have a few things in mind:
New pokemon obviously but I don't expect there will be a great deal of them as it would be a bit much to rig the number of pokemon tally up above or near 600, so I don't think there will be a great deal of new pokemon, esecially due to the fact that the game is due to be released this Fall in Japan, if you recall, the new generations in the past required a lot longer a wait and this was due to the fact that it does take quite some time to design a pokemon and encorporate it's compatablity with the game and past games. Which also might come into play, I see trading being an issue due to the fact that there will be newer pokemon and attacks included within the game so unless they do what they did between Red & Blue to Gold & Silver trading I fear that we may have some trading restrictions with Pokemon Black & White.
I expect to see a more in depth storyline, one that isn't a carbon copy of the previous games just adapted to a new region. I think Pokemon Black & White will be a little darker in some senses and possibley have less landscape to discover but more cities and the scans did give me a reason to think that.
I expect that the gameplay will be wuite different seeing as in it is now in 3D and unlike the 4th generation and HGSS. The overworlds are a lot bigger too, it seems that pokemon are stepping out of the past two dimensional characters and tiles and into a broader more adapted game. Though with other games I've played that are like Pokemon Black & White, the graphics tend to be somewhat choppy. Imagine PS1 on the DS, those sort of graphics but obviously on a smaller scale.
I expect there will be a new route taken with the games, we're in for something different.