Differences From D/P (Wikipedia)

The game features Giratina as its mascot and introduces its Origin Forme (オリジンフォルム, Orijin Forumu), as seen in Giratina to Sora no Hanataba: Shaymin. Its initial form is referred to as its Another Forme (アナザーフォルム, Anazā Forumu Altered Forme in the English localization). The storyline involves a disturbance atop Mt. Coronet, as a portal to the "Distortion World"(やぶれたせかい, Yabureta Sekai)
is opened and Sinnoh's climate becomes colder. The trainers have been
given new outfits to suit the colder climate. The Distortion World is a
strange place in another dimension. The normal laws of gravity and
physics do not apply here.
Shaymin, which was distributed as part of admittance to the aforementioned film, has a new form known as its "Sky Forme" (スカイフォルム, Sukai Forumu), a dual Grass and Flying-type Pokémon. When traded to Diamond or Pearl from Platinum, Shaymin will transform from its Sky Forme back to its Land Forme (ランドフォルム, Rando Forumu) with the use of a new item in Platinum. When the special Regigigas that is given out with a pre-order ticket for Giratina to Sora no Hanataba: Shaymin is traded from Diamond or Pearl to Platinum, it activates an event that makes Regirock, Regice, and Registeel available in Platinum.
Prior to the game's release, silhouette images of five unknown
Pokémon were shown on the official website. These Pokémon were later
identified as different forms of Rotom.An item called the Secret Key was distributed over the Wi-Fi connection beginning on September 28 to change Rotom's forms.
The Battle Frontier, which was first introduced in Pokémon Emerald, appears again in Platinum. In the Wi-Fi Plaza, an arcade area, the player can play minigames with other people over the Wi-Fi connection.