So, I was bored and gave this a shot. Overall I found it quite enjoyable, but there were a few nasty spots that really need ironing out. These are just things I remember:
There is an extremely cheap death in DDZ1, at the end of the first waterslide, if you try to jump to the left at the end, you hit an invisible wall and die outright. Extremely lame. The first spot with spikeballs is practically impossible to dodge the first time as well.
SNZ in general is long and really kinda tedious after a while. It wasn't hard, either - just long as hell. Put some spice in there somewhere. SNZ also has massive consistency issues. In some places the oil is instant kill and in others it's perfectly fine to stand in. That's extremely cheap and should be fixed.
The raising quicksand areas in GCZ are tedious and cheap. The first time you're basically guaranteed to end up in the quicksand, and since it's not instant death you'll sit there for 20 seconds trying to get out onto the ground before dying eventually. The insta-kill slime used later is better than this. GCZ3 is complete BS. The quicksand is quirky and you have to rely on just mashing jump to get out, hoping the step-up code triggers. Also, heaven help you if you try to actually get out onto the middle platform.
Not to be a broken record, but that framerate in Fume Shaft 2 is terrible. I lowered my resolution to 640x400 to try to get rid of the SIGSEGVs and it STILL killed my framerate. It also feels like you basically NEED the Whirlwind Shield to get through this level. Might I suggest a little more leeway in your level design?
The 2D sections in LGZ are really cheap with those stupid enemies everywhere that are impossible to see coming.
Overall, though, the entire pack was quite playable (crashes aside) and just needs a lot less of using the same gimmick over and over in the stage. As guilty as I am with copy-paste in Mystic Realm, this does it a LOT more than me. Seeing the same trick 5 times in a stage gets old.