About Diamond/Pearl
The D/P is the 4th generation of Pokemon. Now D/P was moved to the DS platform with a lots of changes. Not only the region was moved to Sinnoh, but there are much more new features.
Good Changes in D/P
3D Overworld
One of the most important change. Now the map is looking better, and have a sense of depth.
No more ALL-CAPS
All Pokémon names are still written in ALL-CAPS, but most other names are in normal caps. I think the Pokemon games did not store capitalization data so they use ALL CAPS.
Trainers can say in the middle of battle
When you are fighting Gym leaders, you will hear Gym Leaders say stuff in mid-battle.
Contests
The Pokemon Contests was changed, now you feed Pokémon Poffins and the Pokemon Contests are more interactive, and fun, but it's still difficult in Master level.
Wi-Fi
Now you can trade, battle over the Internet! Thanks to Nintendo Wi-Fi Connection (Nintendo WFC).
Safer save data protection
You can't save game if you started a new game with old save data.
Bad Changes in D/P
Surf != Earthquake
Now Surf will attack all 3 Pokemon...
Slow Surf
The Surf HM was slower, in Hoenn, the Surf is very fast.
3D Overworld
The 3D Overworld has some small appearance problems because the sprites scales as you move up/down. Scaling a sprite isn't a good idea because any interpolation can't make the sprite still look smooth after scaled.
Battle Message Scroll
Now the battle message was scrolled automatically, so you need to read faster.
