Step aside Diamond and Pearl, cause here comes Platinum!
The differences are incredible. There's more stuff to do with Wi-Fi, especially the new themed Wi-Fi plazas.
Aside from Wi-Fi, the game has considerably less lag, the graphics for scenery and other things have been improved, and the story has been given some more depth. Although it's darker than some other Pokemon stories, they've added some random humor in there to balance everything.
The difficulty is just where it should be. There won't be too many instances where you'll run into a Pokemon that has a higher level than you expected it to be, and they've increased the difficulty (and puzzles) of certain gyms, giving them more spice, and especially more trouble for you.
Along the lines of the new formes (apparently, Nintendo decided to add an e in there), it's an interesting addition. Although, if you don't have your hands on the event-specific Pokemon... good luck trying to find one.
All in all, this is a pretty solid and fun game. I would recommend it to any Pokemon fan.