The gameplay of Pokemon Pear and Diamond version is a little bit different from the other games, as more futures are availa
be and the use of the Pokebag (menu) is more easy in this game. But that is just a minor aspect why this game have the best Pokemon gameplay ever.
To number the reasons, the first one would be an improvement in the graphics. The game looks more realistic than before and the scenarios have now deep and 3D in their settings. The second improvement with this game it is in the exploration of the map, so far Diamond and Pearl has the biggest scenario in any Pokemon game, more than Gold and Silver where the player could explore two regions at the same time. The third reason would be the expansion in the story and the side quest (giving more replay value, if the player ends the game without completing the side stories). The fourth reason and probably the best upgrade in the franchise is the use of the Nintendo Wi-Fi to let the players exchange Pokemon world wide, surprising the necessity of finding a trade partner as well getting the player to if they have a Wii to use the game Pokemon Ranch.
The sound and BMX were improved as well; they don't sound as monotonous as the first games the fifth and final reason why the gameplay improved so much. That could be why the developers included different weathers on the Sinnoh territory beside the day and night feature that was introduced in Pokemon Gold and Silver, as well the Poketch which is a cool device that let the player follow his Pokemon development.
Because of the previous reasons that's why the Gameplay is a perfect 5, because with a little work and some changes in the game dynamics, the developers made a fantastic game.