Good game design is eternal. The way all of Mario's moves connect so beautifully, with pretty much all of them contributing to a diverse moveset that is extremely satisfying to master. The variety being a natural extension of the main gameplay, with platforming, sliding, and light combat. The compact maps that (For the most part) use their space to the fullest extent. All of this was good in 1996, and is still good today.

Game design is also subjective, and what works for one game may not work for another. A lot of people say the game shouldn't boot Mario out of the painting after getting a star, but this can work better than the alternative of keeping Mario in the level. In Bob-omb Battlefield, Koopa the Quick is at the beginning of the stage, so being booted back is more convenient.

However, such is not the case in a stage like Tick Tock Clock, where the majority of the stars are in a linear set of obstacles, with each a bit further than the rest. This was bad design in 1996 and is also bad design today, and Tick Tock Clock wasn't the only stage with such issues.

I can forgive a bad camera, some spotty visual design, and even short length, but repetitive content I cannot. Don't take this as a knock against the game; It's aged extremely well for such a pioneer. However, some of the mistakes it made have nothing to do with the new dimension it explored, but instead fundamentals that apply in 2D as well.