Well, 5 hours into playing the game, and I've finished the grand prix on 100cc and 150cc, it is way, way too short.
Other downsides? Well, the Single Player is Poor, very, very boring, There isn't enough customisation (3 aspects) to warrant adding it, there are only 8 tournaments to do, which means a piffling 32 tracks to play, and who's idea was it to add the mii's? They sound like 2 year olds. The difficulty also just isn't there unless you opt straight for the 150cc class.
On the upside, playing it on multiplayer is epic, great fun, but the balloon war can get tiresome pretty quickly, as you can't unlock anything through it. You can, however, team up with a friend to do the tournaments, meaning you both unlock things, all you have to do (if you're not competitive) is make sure one of the players wins.
(It's great fun if you are competitive though)
Graphics, sound, controls are all pretty solid, nothing extraordinary to report, but better than some other karting racers out there. Having it 3ds only seems a bit odd, there's not really enough 3D in it.
Overall, 2 sides of the extreme, on the one hand, it's a properly fun multiplayer game, but play it on your own, and you'll be trading it in within a few hours.
Was expecting more, but it is the strongest Mario Kart to date, but again, that's not saying much. A few more diverse tracks would have been welcome, there are mirror tracks, but in my eyes, that doesn't count.