To me, the gameplay doesn't seem very polished. For example, the mushrooms are too fast. While in the original they only come out of the block for about a second, here they "run" immediately. This has some consequences in the gameplay that make the game much more difficult. For example in the levels without solid ground. Or the 1Up mushroom in level 1-2, which you can no longer get on the ground.
The behavior of the fire flower also irritates me. The fireballs don't hit the ground immediately, but make an arc first. This makes it much harder to hit the piranha plants.
I also found a bug with the Koopas. In 1-2, the shells get blocked at the edge of the screen instead of moving on when I kick them away.