Super Mario 3D Land is a good game that excellently fit the platform it was on.
Yes, this game is vanilla. Out of any 3D Mario, this is the one that feels the most "on-brand". It lifts its structure from 2D Mario, focuses on standard platforming with no wacky gimmick (well, mostly, we'll get to that), features only one new power up, and its central power up is a nostalgic returning ability.
But you know what? Vanilla is a good flavor. And when every other 3D Mario goes in bolder directions (even 3D World, the direct sequel, focusing on its multiplayer and wider camera views), gimme some vanilla in my bowl. You got solid platforming in bite sized levels that you can pick up and play. Movement feels good, if a bit simplified compared to the other 3D Mario games. The powers are as solid as ever, nicely translated to a 3D platformer. It just works - simple yet elegant, a bit too easy and lacking in boss variety but a blast to run through. For a portable game, the structure worked perfectly, even if it was not as ambitious as its cousins.
If 3D Land has one gimmick, its the usage of 3D, some of the best on the 3DS. The effect helps with depth perception in platforming, adds a new way to structure puzzles, leads to some nice effects in certain levels, and is also just dang nice looking. Lots of games had good 3D at launch, but 3D Land sells that effect for the gameplay and the presentation.
It makes perfect sense why it was a 3DS killer app. It's a quintessential Mario game that is approachable as heck and sells the 3D well. It may not impress compared to the others, but for what it is, it's a good time!