Great game, but beware:VERY addictave.
New positions for the pokemon sprites.
much better animations for the pokemon, especially if you send out the pkmn
2 new characters
much bigger Sinnoh Pokedex(210 pokemon)
better storyline
great entrance scene for Giratina.
Distortion world
battle park
battle house
Heatran has a small story
new forms for Rotom, Shaymin, and Giratina
more WFC options!
Those are the main new things, of course, the pokemon "recipe" for a great game has not lost its charm, of course remember that this game takes more than 100 hours to complete it 100%. But buy it, whether you are new to the great creatures, or whether you know pokemon already. 493 pokemon to catch, though only 485 are needed to complete the national pokedex, which is one of the 5 objectives. The story, as i already mentioned, is also a little more exciting than previous pokemon games. i won't spoil it for you, but the distortion world is quite exciting, but also a bit like a giant puzzle.
also, as i mentioned, 210 sinnoh pokemon now. As usual, there are a few pkmn that are only found in Diamond and Pearl, such as stunky and glameow, so make sure you've got a friend with D/P.
Pokemon is a highly addictive game, making you always go back and play to at least 40 hours. The main storyline takes around 12 hours to complete.
Another new thaing are different gyms. They themselves are a giant puzzle, for example the 2nd gym, in which yo need to defeat traniers to make the giant floral clocl's handles turn, so you can reach another traner, and so on untill you reach the gym leader.
All in aall, platinum is an excellent pokemon game, well worth your money.