What I'm going to type up here is mostly in response to the graphics I'm seeing, since my laptop is currently down and I cannot actually play the hack. but what I'm about to point out also has gameplay applications. If you're able to stick with me here, let's go into detail on what I see wrong here.
vhttp://bin.smwcentral.net/u/23281/test_00014.png
This image in particular is a telltale sign of my biggest issue with what I'm seeing. This whole screen just doesn't seem to flow well; rather than acting as an extension of the landscape, what we're seeing is something that is purely deviating from the theme of the level.
Your graphics here feel gray and dull, clashing with the vanilla backgrounds and sprites. The skulls(hurt blocks?) don't look like they should hurt the player like they're clearly supposed to. All in all, your hack feels detatched from itself in many ways, seeming more like a mish-mash of miscellaneous ideas than actual levels.
One thing that you've neglected to think about is the level theme and how it effects the layout of your levels. One should constantly be aware of exactly what they're making is meant to feel like.
http://www.nesmaps.com/maps/SuperMarioBrothers3/SuperMarioBros3Map1-2BG.png
Look at how the terrain is handled in world 1-2 of Super Mario Bros. 3. It goes up like a real hill, be it a steep hill or a small, steady rise. It looks like realistic terrain but at the same time still smoothly curved for the most part. In these kinds of zones, even the straight paths have a bit of bumps in them, much like real earth. The actual flow of the level is heavily impacted by this in level design terms, using these slopes as obstacles or perhaps tools for the player.
Now lets look at zones with man-made floors.
http://www.vgmaps.com/Atlas/SuperNES/SuperMarioAll-Stars-SuperMarioBros3-World1-Fortress.png
Entirely different scenario here. Ground is more flat, slopes(if they can indeed be called that) are not smooth, but rather jerky. This also impacts level design in many clear ways to impede and assist the player.
A great example of fitting terrain changes that I often uses as a reference comes in the Sonic franchise.
http://img210.imageshack.us/img210/1079/icantholdalltheseloops.png
Look at all these loops. Each one from a different type of zone, and each one with its own design. Some designs are similar to others (Green Hill to Palmtree Panic, Emerald Hill to Angel Island), but there's ultimately enough differences to make each loop unique. Arguably even more important is how each one effects gameplay differently depending on the zone that they've been placed in, and are structured differently.
Your levels seem to lack this sort of continuity on the other hand. Rather than being built around their theme, they're built separately, ignoring the theme altogether and failing to achieve that continuity that glues everything together, turning it into a mish mashy mess.
I also wanted to mention, that though you may have read it already, you can never re-read APPALLED too many times. I find that it really helps me when trying to think of ideas for levels.