New Pokemon, Updated graphics, and online play are just a few of the reasons you should buy Pokemon Diamond/Pearl.
Story
The storyline hasn't changed much since the Game Boy days of Red and Blue. You go through a string of events that leaves you with one of three starter pokemon: Piplup (Water-Type), Chimchar (Fire type), or Turtwig (Grass Type). You then must set out on a journey catching and raising your own pokemon, all the while taking on the 8 gym leaders and hopefully the pokemon league. It's a true cliche.
Battle System
Pokemon is an RPG game, so it has many RPG elements to it, such as learning new moves and leveling up. Every pokemon has unique stats from their HP to their attack power. Many refer to Pokemon's battle system as Rock, Paper, Scissors. In many ways, that's true. Every Pokemon has their own type and each move has it's own type. The pokemon's type will determine what move types they take 2X damage against, and which moves their strong against (Example, Piplup, a water type, will take 2X against Grass Type moves, like Razor Leaf. However, Piplup will only take .5X damage from Flamethrower, a Fire type move). I could go into more detail, but for the sake of the review, and your interest, I won't. Although this system requires heavy strategy, and should whet any RPG gamer's appetite.
New Pokemon
This is what truly keeps Pokemon gamers coming back for more. This time, there are 107 new Pokemon, bringing the grand total to a whopping 493 (including some secret ones, shhhhhhhhh). They've also added some extra features involving certain GBA Pokemon games that will help you get pokemon that you otherwise could never get. I won't go into detail since you must beat the game first.
Multiplayer
The Multiplayer has been enhanced from previous versions of the game as well. In addition to being able to trade and battle your pokemon via local Wireless, you can now trade and battle pokemon around the world via Nintendo Wi-Fi connection. So if someone puts a Lv.100 Jirachi up for trade 10,000 miles away and they want a Lv.20 Pidgey. You can make that trade and get a Jirachi for a Pidgey (also, anyone who trades a Lv.100 Jirachi for a Lv.20 Pidgey who a sucker!).
Graphics/Sound
The environments have gone 3-D for the DS Pokemon, and the 2-D characters fit in well, oddly enough. The sound feels overly generic, and in my opinion is sub-par.
Overall
This is one of the greatest games the DS has to offer.The overall quality of the game is great, and if you weren't a fan of Pokemon before playing this game, you will be:)