Addicting. As. Hell. :O
Gameplay - The overall concept of Pokemon RPGs is exactly the same as it's been since Red and Blue. Playing as a male or female Pokemon trainer, one must travel around the region (In this case, Sinn'oh) from city to route to city, battling Gym Leaders for their badges and, most importantly, catching 'em all! Once one gets all eight badges, he/she must go through Victory Road to the Pokemon League, where he/she must battle the Elite Four and the Pokemon League Champion to claim the Champion title.
Same thing as the others before, right? Well, it's still as great as ever, with a bunch of new features added into the game. Pokemon Contests are re-introduced from Ruby/Sapphire, and all take place in one city in one contest hall instead of four contest halls, one in each of four cities.
You can literally "Catch 'em All" in Diamond/Pearl. All 493 Pokemon are able to be part of one's collection. A several new evolutions are available for old Pokemon, especially the Gold/Silver/Crystal ones. With a new 106 Pokemon added to the Pokemon world, there's no end to the fun! :D
Battles are great. there are a bunch of new moves to be learned, there are more type combinations, and other Trainers will occasionally fight with you in Double Battles. Pokemon game battles are the only way one can even get through this game, of course, since one needs experience and money to make it through.
One of the most exciting features of this game is going Wi-Fi, like every other DS game. This is awesome for Pokemon because people all over the WORLD can now trade and battle with each other! That's just awesome.
And those aren't all the extra features in this game. Play it to find out the rest and understand the awesome gameplay of Pokemon Diamond/Pearl.
Graphics - It's a pseudo 3-D look this time around, and it looks pretty good. Everything is very nice, Pokemon look really good, and nothing looks out of place. Just amazing.
Sound - The music is great, even though it's on a handheld. The Pokemon tunes, although different, all still have that same great feel as the games before.
There is SO MUCH MORE to be said, but it would take me too long to type it all out, and I could spoil too much info if I did. Just get this superb game and play it. Only then can you really feel how freakin uber this game is.