Don't clump the mines. Doing "Every other half-space" works MUCH better, and often looks much better, too.

Less is more when it comes to enemies. One well-placed enemy often beats an arsenal of badly placed enemies.

Tilesets need aesthetical appeal, flow, and most importantly: A Theme. Pick something and stick with it.

Random/Semi-Random "#5" tiles usually look bad.. stay far, far away.

While sometimes Difficulty and Repitition are great by themselves, mixing the two are VERY BAD. Example: The mines and gold "waves". It just doesn't look good.

Symmetry is a great way to improve the looks of your maps, and often makes them much more playable.

Odd and often overly difficult enemy placements to try to make a level challenging is a big No-No. Instead, make the player try to beat his/her own personal best, rather than struggling to complete your one level. Remember, they dont NEED to play your level. Treat them to something once they've decided to.

Thats all the help I can think of at the moment, regarding this map.

Cheers,
~Mosh

Bad tileset, mine clumping, near unplayability, an impossible switch, bad enemy placement, and shabby level concept.
While it isn't completely unplayable, and while I did like the mini-platforms above where N starts, the majority of this level isn't even close to being a few notches below par. It's sadly lower.

But the floorguards worked well with the Gauss. =D (no really, I liked them. ^_^)

Cheers,
~Mosh