It demonstrates 3D dungeons, 3D minigames, and on top of that, everything you've already played through in Diamond And Pearl! And I cannot tell you how many times I've have to spell the word 'Diamond' today! And I think almost everytime I spelt it wrong the first time round! Yay! 

It looks cool, sounds great, and probably plays better than it looks, I can't wait.

Source.

Edit: I regret to announce that the video file format wouldn't comply with YouTube at this time, until we can rectify this issue please use the souce's video player. We are sorry for this any inconvinience this may have caused.

Edit 2: A Japanese friend has looked at the Pokemon Platinum Trailer and has said that according to the video the Japanese Release Date is September 13th.