Doesn't stand out very well anymore since the whole concept of being a 3D Mario game that uses 2D Mario level structure has been done in the exact same way, but better, in 3D World. But it's still a Mario game and as such has the pure fun, polish and joy that comes with it.

It could benefit from a few more unique level themes though. Too many grass levels, snow levels, ghost houses etc. There's definitely a few that stand out, like the blocks that vanish in time with the music, the clock/cog level, and the buzzsaw level.

The bosses might be the weakest aspect of the game. There's a grand total of 3 that are reused for all 8 worlds (or 16 counting special worlds). Boom Boom and Pom Pom especially are pathetically easy. Bowser himself does actually do a pretty good job at transitioning his classic 2D battles on the bridge into a 3D design, but again, it's reused too much. The final Bowser fight on the other hand stood out a lot more, acting as a kind of platforming/boss hybird.

I also like that after beating the last boss you unlock an entire hard mode which can be either more enemies, a strict time limit, variations on the stage hazards, or a shadow Mario that follows your movements allowing you no rest. But what's kinda weird is that it doesn't just use the same levels once and make them harder, some stages get reused what feels like 3 times, which means others don't get a hard variant at all.

And the reward for 100%ing the game is a level to challenge even veteran Mario players, something that is standard for all Mario games now, but at the time was pretty new. For the most famous example of a casual series, Mario games can really throw some tough stuff at you if you unlock it.

Basically it's a 2D Mario in 3D. It's fun.