I hate the 100% requirements. It's really hard for me to tell the difference between the coin colors and trying to just get all of them is a pain. Needing max health at the end of the level is just annoying, so often would I get hit by something small like a watermelon seed at the last minute and have to redo the level. Also a lot of the flowers only spawn from killing all the enemies in a room or finding invisible item clouds. It generally requires a much slower pace than what's needed for getting all hidden exits in Mario World.
Even though I normally love to do the special world stages in Mario games I eventually gave up unlocking them in this game because it wasn't fun for me. However, once I just started playing it any% I found the game too easy and short.
Otherwise, the art style is great, some music is iconic, and a few levels feel innovative for the time. I could do without the baby screams being so shrill though. I understand it's a baby but it got on my nerves quickly. It's a fun game but has some major problems for me.