Object placement follows the same kind of idea as enemy placement: what objects will you use and how will you use them? It always helps to have gameplay over aesthetic value, but when you have had practice you will learn to blend both together to create a pleasing experience for yourself and for others.

Tilesets. These are tricky. Do you build a tileset to fit your objects or do your place your objects to fit your tileset? It may sound like the same thing at first but they are two very different things.
Some of the best authors are able to blend aesthetic enjoyment and wonderful gameplay out of their tilesets. Enemy and object placement play a huge role in how a tileset is used. Many people have ruined good tilesets with horrible object/enemy placement. Many people have also made terrible tilesets and had great object/enemy placement to go along with it. Learning to blend them together takes time and practice.

You've probably read it a few times already, but practice is really the best way to become better. Not everybody starts out amazingly good. If they do then they have probably been mapping for a while before they joined NUMA. The best thing to do is to play maps you enjoy and pay attention to how the author built their tilesets and placed their objects and then trying to build something using ideas from many other maps to create a unique map.

I hope that helps. I didn't know how to be extremely specific so I just gave you my point of view on the main aspects of building a solid map. :D

Alternately, don't be afraid to use tiles if objects aren't working for you.

When working with enemies, there is but one thing to keep in mind: Enemy placement. Nothing else matters. Enemy placement should offer a good challenge as well as an element of the player knowing they can avoid the enemy in question if they are quick and agile or slow and steady. Maybe a little of both. If when playtesting, an enemy you placed doesn't provide enough challenge or is far too hard, place it elsewhere in the map, replace it with a different enemy, or remove it entirely.

Enemy choice is another important aspect of mapping. Having the right kind of enemy fit the map you're making is one of the most difficult decisions I usually have to make. Often I will build a chaingun map and instead try it out with zap drones and rockets, lasers and rockets or gauss, gauss only, lasers and chainguns, etc. Some people know exactly which enemy would be perfect for their map; in this case, they need to have the enemy placement to create the right kind of atmosphere. Some people know the perfect enemy placement for their map but lack understanding of what kind of enemies to use. It goes back and forth and takes practice to really learn these concepts.

I've believed for a long time that style is detrimental to an author's repertoire. Many people disagree with me but I find that a style leads to people calling every map you make a rehash of the old one. If you can manage to have a style that doesn't impede on using new techniques or new ideas in your maps and keeps them fresh while still having that touch of your own personality, that is what makes an author shine.

As far as individual mapping goes, I find it is always best to constantly play maps and constantly rework the maps you make. I spend a decent amount of time rebuilding different portions of my maps to try new things and come across something interesting. A lot of the times a fun map comes from constantly developing a section until it works just how you want it to.

There are maybe a few things you should be sure to remember:
Don't be afraid to use objects! If a tileset doesn't give a desired gameplay effect, try building your idea with objects. Bounceblocks, one-ways and even trap/locked doors can add a new element to how the ninja is allowed to move throughout the map.

This will be continued in a new comment. (1000 word maximum allowed for comments, don't want to go over.)

All my mapping right now is being done for other things. I'll let you know when I can collab with you. :)