I think the main issue with this right now is that you don't use poison mushrooms well. I mean heck, they seem to be even more common than regular mushrooms. The big flaw with this is that it actually discourages exploration, which stunts its replay value. If you were more open and honest about them, basically always telling the player "Hey, there's a poison mushroom in this block, open at your own risk." Then it could be made into an interesting risk/reward system, placing them where the player can make some other gains, but at the cost of making the environment a little more dangerous. That would be one way make it a far more interesting mappack, and giving it a much needed mechanical thread.
That said, it does have some interesting moments. I particularly liked the pipes in the opening where you had to avoid the red koopas walking past. Another good moment was when a stack of coins led directly into an invisible poison mushroom block, which actually gave me that idea I talked about earlier. The 1-2 warp zone was also really well implemented, and actually made me think about the space. Having dual-purpose levels like that would also be a good thread to pursue.