Party Karting. Kartying?
To start with, I hadn't really had much interest in playing Mario Kart again. I played the hell out of earlier entries, and well, the series hasn't exactly reinvented itself in the last decade or so. But everyone seems to agree that Mario Kart 8 is fantastically fun, and they're ... mostly right!
I don't know how it compares to Mario Kart 7 (which I never tried), but by now, 8 has become fully optimized as a party game. You no longer have to unlock engine classes or features; unlockable characters and items mostly share statistics with the defaults; and even the unlocks are trivially easy, a simple matter of winning cups (it doesn't matter which) and collecting coins. Tracks don't really have shortcuts or other secrets to learn; after the first lap, you're basically an expert.
This isn't to say that there's absolutely no skill involved in the game, as clever tactics and extensive track experience will always yield a better performance. But the bottom part of the curve has essentially been chopped off. In a party setting, this is a really great thing, as everyone is on a fairly level playing field -- and since there's nothing vital to unlock, you can enjoy the breadth of the game right out of the box.
EDIT: Since this post, I've ventured back into Mario Kart 8 a couple times, and - surprisingly, to me - there are tricky shortcuts hidden throughout many of the game's tracks. They tend to be of the rough-terrain variety, e.g. you can only really take advantage of them with a mushroom or a power star -- which is exactly the kind of gradually-acquired track knowledge that lends the game further depth. So that's pretty rad. To return to my original point, there is a lack of scale in unlockable content, which is nice for some instant fun...
But these aspects interfere with the game's longevity, since there's little motivation to keep unlocking things, or explore tracks further. Outside of a party setting, MK8 doesn't last long.
So, its appeal is highly contextual. At its best, the game is some great fun (and to its credit, the online play feature is surprisingly well-implemented). It just doesn't have much staying power.
Better than: Mario Kart Wii
Not as good as: Mario Kart 64, I think, although this is becoming more difficult to remember accurately.
Seriously, I'm amazed: at how seamless and painless the online play is.
Progress: Swept the 100cc cups, unlocked all the characters.