Biggest 3rd version upgrade, it's almost a remake
>Graphical updates in most if not all areas, some fairly major
>Game is faster in general, surfing and battles are faster
>Some Gym layouts have changed and the order of the Gyms is different.
>Looker and Charon added to the storyline
>Expanded Team Galactic plot with distortion world
>Pokemon have animated back sprites
>Important trainers such as Gym Leaders, Tag Partners and Galactic Admins now have animated sprites
>The battle background changes depending on where you are fighting, e.g. the Cynthia has a different battle background
>Before entering the Fight Area, you will end up in a double battle against Volkner & Flint
>Battle Frontier added
>You can rematch your rival and Gym Leaders
>You can own your own villa which other characters will visit, and buy furniture for it
>Updated battle animations for some moves (apparantly)
>Articuno Zapdos and Moltres are catchable after talking to Professor Oak in Eterna City
>Expanded Sinnoh dex with Pokemon that weren't in DP, and updated trainers using them
>There were a large variety of new WiFi features (not useable anymore), including Wifi Plaza which had minigames
>Vs. Recorder was introduced. (available in games afterward)
>New move tutors
>Rebattleable trainers in Pokemon Centers each day
Here is a quick example of how some locations were done up in Platinum
