Well, D/P is obviously very similar to the previous games because keeping a franchise practically the same and still getting people to buy new games is what Nintendo is best at. Still, there are quite a few new things in D/P, which include:
- 4th Generation of Pokemon (obviously)
- New moves, items, etc. (again, obvious)
- Use of the DS' touch screen capabilities for certain things (though most of the game is still played the old way)
- Semi-3D maps, again thanks to being on the DS
- Ball capsules and stickers
- Pokemon trading without a linking device of some kind
- GTS trading center, where you can trade your Pokemon for someone elses from across the world (the downside being that it already has to be in your 'Dex)
- WiFi Battle Tower which can be used to battle friends and trainers from across the world
That's all I can think of off the top of my head. That should pretty much cover it, though.