Number 5: The main character.
Now look... JUST LOOK! doesn't that look better than this?
Number 4: Instead of an "Elite Four", There is an "Elite 100!"
Yeah, this one's kind of a stretch, but in both games there is a place called "Mt. Battle" which is basiclly a test of your Pokemon strength. In Colosseum, if you complete it in both the Story Mode and Colosseum Solo, You get a Ho-Oh from "MATTLE", and if the same happens in XD: You get a Deoxys.
Number 3: "THEY'RE ALL CONNECTED!"
Listen: When I say that they're connected, I mean in Relation and Story. The Main Character is a FATHER and this is his son, Michael Making Colosseum the FIRST playable father in a Pokemon game!
Number 2: The Music
Now these are my 2 favorite songs from Colosseum.
AND NUMBER ONE: GLITCHES and Changelings?
Bronies know what I mean by "Changeling", but for you non-bronies a "Changeling" is basiclly a Spy from TF2, but a succubus as well.
And before I forget, here's the intro: