Of course, the most important thing about learning the differences in a Pokemon version is knowing which pokemon you can and cannot catch. The thing that will probably make all Diamond and Pearl players jealous is the fact that, not only can you catch the awesome Ghost/Dragon pokemon Giratina, but you can still get the featured pokemon from Diamond and Pearl. Naturally, that's Dialga and Palkia, who are both catchable in Platinum.
Giratina still has its Altered form from Diamond/Pearl if you encounter the pokemon outside of the Distortion World. In the Distortion World you can see its Origin form. You can also use the Platinum Orb to get Giratina to change forms while outside of the Distortion World.
Giratina's Origin form has a different ability, Levitate instead of Pressure, and its stats are very different. Whereas the Altered form was better at Defense and Special Defense, the Origin form has a better Attack and Special Attack stat.
Shaymin now has a Sky form, which you can see by using a Gracidea Flower on it when it is unfrozen during the day. The Sky form has worse Defense and Special Defense, but better HP, Attack, Special Attack and Speed.
Rotom now has five new forms that make him look like household objects: Frost Form (Refrigerator), Spin Form (Fan), Wash Form (Washing Machine), Cut Form (Lawnmower), Heat Form (Oven). When you open up the box that Platinum comes, one of the inserts on top of the instruction manual will have special instructions that show you how to get the Secret Key that unlocks your Rotom's new form. You will have to trade or compare notes with friends in order to be able to get the other forms.
In each of these forms, Rotom learns a unique move that he normally doesn't learn. Frost Rotom learns Blizzard, Fan Rotom learns Air Slash, Wash Rotom learns
Hydro Pump, Mow Rotom learns Leaf Storm, and Heat Rotom learns Overheat.
Manaphy is still uncatchable in the game, but you can add it to your Pokedex by looking at its picture in a book in the Trophy Garden. You will need to do this in order to get the National Dex.
After getting the National Dex and talking to Professor Oak in Eterna City, the Three Legendary Birds, Articuno, Zapdos, and Moltres can be encountered around the world and can be caught.
If you got the special Regigigas from the Toys R Us event, you will be able to find the Three Legendary Golems, Rigirock, Regice, and Registeel, hanging around Route 228, Mt. Coronet, and Iron Island respectively.
You can still get the non-event Regigigas in game the same way as in Diamond/Pearl.
Dialga and Palkia can be found on Spear Pillar at different times. You will need to talk to Cynthia's grandmother in Celestic Town and find the Adamant and Lustrous Orbs in Mt. Coronet in order to see them and catch them.
Cynthia will give you a Togepi egg after you defeat Team Galactic in Eterna City.
Bebe will give you a level 20 Eevee when you arrive at Hearthome City.
In Veilstone City, a man will give you a level 25 Porygon.
You cannot get the following pokemon without trading for them:
Starter Pokemon from previous Pokemon games (Charmander, Squirtle, Bulbasaur, Chikorita, Cyndaquil, Totodile, Treecko, Torchic, and Mudkip families).
Trapinch and its evolutions
Mew and Mewtwo
Raikou, Entei, Suicune, Lugia, Ho-Oh and Celebi
Latios, Latias, Groudon, Kyogre, Rayquaza
Manaphy, Jirachi, and any Deoxy forms
For the Fighter/Trader in You
Platinum features a lot of useful tweaks to its online and battling systems that will really make your life easier if you love trading and battling over Wi-fi.
GTS, the Global Trading Center, now has an email system that really sends messages to whatever email you specify. You can use this to receive emails whenever your trades complete.
There is a new key item, the VS. Recorder, that records and saves your Wi-Fi and Battle Frontier battles. The videos can be uploaded to the GTS where your friends can download and watch them. You can also use this to trade pictures.
There is a new Wi-Fi area called the WIFI Plaza where you can interact with up to 20 other people and play lots of different mini-games. It can be found at the basement of the PokeCenter.