There aren't really any objective problems with it. It's incredibly simple and the only parts of it that don't use exclusively global enemies are the underwater section and the part immediately after it, with the paragoombas and parabombs.
It also makes light use of custom graphics, though not a whole lot. The main character is changed to Luigi, coins are given a completely different look, and the "egg" at the end of the level is uh...there.
The design isn't bad either. Most of the enemies actually have a purpose and don't just go right over you, and you can't just jump over most of them.
Overall, it's pretty decent, but you should look elsewhere for a more meaningful hack.