For those of you who didn't know already, Platinum has already been released in Japan. So expect to hear a few good things until it comes out in English(spring next year). Also for those of you who wished certain aspects of the game to be changed, the game is already released so things won't be changed from the Japanese to English versions.

Here are a few 100% certain confirmed details

Pokemon Details
1. Giratina will be able to transfer to Origin Form outside of the Torn World with the Platinum Orb and the same goes for Shaymin's new form with the use of the Glacidea Flower
2. Rotom's new form can be changed with the use of the event item Secret Key(yet to be distributed)
3. Regigigas will be changed from the D/P level of 70 to a miniscule level of just 1
4. The legendary birds Articuno, Zapdos, Moltres(not lugia or ho-oh) will be obtainable at level 60 as roaming pokemon like Mesprit and Cresselia
5. The other three regis can be caught with the use of a Regigigas event at lv.30
6. Eevee can be obtained at Bebe's house before the national dex, a man in veilstone can give you a porygon as a gift, Cynthia gives the player a togepi egg once the Galactic Building is cleared at Eterna City

Gameplay Changes
1. The new Battle Frontier will feature Battle Tower, Battle Factory, Battle Castle, Battle Roulette, and Battle Stage along. Move tutors will also be available for use(virtually every dragon type pokemon will be able to use the move Outrage with platinum move tutor)
2. Towns, cities, and dungeons have been mostly refurbished
3. Pokemon animations have been changed

NOTE: The new sinnoh dex has added pokemon giving the total of 210 pokemon(instead of the D/P 150) to be seen before you can access the national dex and Manaphy must now be seen in order to obtain the national dex(look for its picture in Trophy Garden if you don't have one and yes, manaphy is still uncatchable without an event)

Thats just a crash course in the new updates. If you have more questions or comments about the game, feel free the post here. I'll be more than happy to clear up a few details