->The Pokétch has been redesigned. Players can now move up and down between apps instead of having to go through all of them to return to a previous one.
-> The Eterna, Hearthome, and Veilstone Gyms have all had complete redesigns in the interior.
-> All gyms now only have Pokémon of their type specialty.
-> Legendary Pokémon (e.g. Uxie) will reappear where they are usually found after a week if they have fainted in battle. However, they will not appear again if they have been caught.
-> The battle menu was redesigned.
-> The player meets Professor Rowan and the opposite gender character earlier, and obtains his or her starter Pokémon on Route 201.
-> The first battle is against the player's rival, who, as always, has chosen the starter Pokémon that weakens that of the player.
-> A new area known as the Distortion World (Japanese: やぶれたせかい Torn World) is playable. It has floor-like sections floating at ninety-degree angles from each other, creating an effect similar to the art of M. C. Escher. It doesn't follow the Laws of Physics, making things like surfing down a waterfall going up possible. The camera angle switches between perspective and isometric views to suit the angle of the player. It also has moving platforms with jumpable gaps between them. Giratina, when here, will be in its Origin Forme and is also the only wild Pokémon located here.
-> Some of the Gym Leaders have been reordered: Fantina is now the third Gym Leader, Maylene is the fourth and Crasher Wake is the fifth. The appearance of players' gym badge case stays the same though.
-> When first visiting the Resort Area, the player receives a villa. Inside on a table is a catalog that lists items that can be purchased to furnish the house in the same manner as decorating a secret base. The player will be visited by NPCs such as the Sinnoh Gym Leaders, the player's mom, Professor Rowan, and the rival.
-> There is a new key item called the Vs. Recorder (given by Looker when the player first meets him in Jubilife City), which allows players to record battles in Battle Frontier and Wi-Fi, and share the records with other players over the GTS. Players can also take pictures of their PC boxes and share them with other players in a similar fashion.
-> The Sinnoh starters and their evolutions are now allowed in an expanded Amity Square, making a total of 20 Pokémon allowed in the Square.
-> The player can now challenge other trainers in certain Pokémon Centers. The trainer(s) are located in front of the desk. These trainers change location every day.
-> The price of the accessories at Pick a Peck of Colors Flower Shop on Floaroma Town has been reduced.
-> 59 more Pokémon have been added to the Sinnoh Pokédex to give it a grand total of 210 entries, with all non-legendary Pokémon introduced in Generation IV now included, along with their pre-evolved forms and several other Pokémon. The added Pokémon, such as Rotom and Giratina, are now available prior to the Elite Four challenge.
-> The legendary birds can be found as roaming Pokémon after the National Pokédex is obtained. They roam Sinnoh much like Mesprit and Cresselia do. The player has to talk to Professor Oak in Eterna City to trigger their appearance. They are all at level 60.
-> Cynthia now gives the player a Pokémon egg containing Togepi after the player has defeated Team Galactic at the Galactic Eterna Building, replacing the Happiny Egg previously obtainable in Hearthome City.
-> Bebe now gives the player an Eevee before the player receives the National Pokédex; Eevee can now be obtained from the first time the player arrives in Hearthome City. It is at level 20.
-> Much like the third version of the last generation, a new Battle Frontier takes the place of the Battle Park. It has the Battle Tower, Battle Factory, Battle Castle, Battle Arcade and Battle Hall as venues.
Fonte:
Bulbapedia.