First of all, each racer now has a plethora of rides to choose from: three karts and three bikes each. These are available from the start of the game. Also, the CC classes return, but with a small twist. The 50CC bracket is for karts only, the 100CC bracket is for bikes only and the 150CC is a mixture of both. IGN notes the difference between the two as the bikes having a tougher time with turns, but being easier to boost with (tip the wheel back to do a wheelie, hold it then zoom off). As for karts, they'll get boosts from powerslides, providing more boost depending on how long the player holds it, eliminating the need to slam on the d-pad to generate these anymore (we're not sure if this means no more snaking or not). Stunts will also provide boosts, allowing a player to perform a mid-air stunt off of a jump and shoot off once they land on the track.
The playable characters in the demo build included Baby Mario, Baby Peach, Toad, Koopa, Mario, Luigi, Yoshi, Princess Peach, Wario, Waluigi, Donkey Kong, and Bowser. There weren't any kind of locked character slots, however IGN noted the character selection screen seemed kind of open and might have some hidden folk to unlock come time for final release. We wouldn't be surprised. Also, Nintendo confirmed that you can in fact race as a Mii, however the demo build did not have that incorporated (we wonder how many times we're going to have to race Hitler online?).
Now, onto the good stuff: online play. As previously reported, the game will allow 12-player online racing. What's great about this is you can hop online alone, or you can have a friend join you through your Wii. Also included in the game's online feature set is the Mario Kart Channel, an in-game feed of the leaderboards and other pertinent information relative to the game. This will also allow players to download and upload ghost data , as well as tournaments and weekly challenges. Sounds fun!
Overall, there is a lot of information to digest here. This game's scope is going to be much larger than anything we're used to in the franchise. And, for that, we're extremely happy.