The improvements in Mario Kart are similar to existing sequels. 16 new courses with 16 courses from previous installments. There is a very nice graphical overhaul, colors are crisp and there is slightly more detail in the newer courses.
Biggest issue I have with the game is the AI/Multiplayer where blue shells still wreck havoc. The AI will relentlessly nail you with a blue shell or lighting bolt on the final lap. In MP matches most people farm an item and hang in the back until it's time to move in for first place on the last lap. It gets frustrating because there's no point in trying to take the lead as the item system punishes you for it.
There isn't much use of the 3D effect in the game, nothing really pops out at you outside of the win screen. The few new items they introduced are lackluster at best. The game really shines with the new courses and kart customization. If you are a fan of Mario Kart this is a must buy, if you never played Mario Kart before this is easily the best arcade racer you can buy.
3D graphics are so so but the gameplay is a blast and it has online world wide multiplayer via wifi. It's a great for casual gameplay which is what a 3DS is. This is one of those games worth owning for the online play. Only downside is the online play takes too long to start. You even have to watch someone else play until it's your turn.