This game is very likely the best MK game ever. And that's really something, since the others (particularly MK:DD, IMO) were all excellent. I originally thought that this game would not have much in the way of innovation, and that the only new stuff would be the new tracks. But now that I have the game, there are a zillion new things in it that are totally awesome, like the Mission Mode where you can fight bosses, the enhanced Battle Mode that can (finally) be played in Single-Player Mode, the wide selection of karts, and many other things, including stuff that it would be spoilerly to mention. I also like the inclusion of all those old tracks from the previous MK games.
As for the online play, I don't know completely how it works, since I can't use it at the time (I wish we had SOMETHING better than Dial-up...).