One thing that appears to be a bit of a struggle for new mappers is the concept of the game world being mostly just an illusion. This is a bit of a concept jump for some people.
Some folks are happy to be absorbed into the game world and never question how it's put together. So when it comes to mapping for the first time and they want to build a cliff edge with a horrific drop they build a 2000 foot cliff actually within the map.
Its an easy mistake to make.
That's another thing us mappers have spoiled for ourselves. That was amazing... how did they do that? We look for the edges of the map so to speak so that the reality intended for us to enjoy never really kicks in unless its totally engrossing.
I don't remember looking for the map edges when playing Theif 2. This was mainly because I was hiding in a cupboard praying the mechanical creature with the creepy arsed voice didn't find me. (If u havnt played theif 2: the metal age then you're a disgrace to the gamer name, go play it, love it and buy new underwear when your finished).
Theif 2: Amazingly pant fillingly scary but equally funny as hell
So... the bottom line is, the better at illusion you become, the more fantastical maps and amazing game events you will be able to create.
3D skyboxes are a godsend for this kind of work. You can create a world beyond the edge of your map that has a realistic look and feel to it and can also include lights, fire, fog... pretty much anything you need.
That 2000 ft cliff we mentioned earlier becomes far easier to create with a 3d skybox.
Or how about adding a huge floating space craft hanging over your map? Done in two minutes... well maybe three if you add wing mirrors.
The most useful tool in you're arsenal of illusion is sound. What the ear hears the brain forms an image of.
Take this scenario as an example. The player walks up to a set of lift doors and presses the call button. They hear an electrical explosion above them, a wrenching sound of scaping metal and a screeching sound heading down towards them. A huge bang then occurs behind the lift doors and their point of view shakes.
Then you hear the familiar ding and the lift doors slide open revealing a twisted mass of mashed metal and debis.
In my map the lift didn't go anywhere. The mashed up lift was always sitting behind those doors but through a combination of sound and effects the illusion is that the lift plummeted down the shaft. Sweet!
Sound is a fantastic way to flesh out the world because almost subconsiously your brain fills in the rest of the unseen world.
A map without sound will always feel empty regardless of the detailed mapping work.
Just remember that when it comes to the physical world you create, what the player can't see, you don't need to build and what the player hears becomes the reality of what's around the unseen corner.
Pretty soon we'll all be a bunch of budding David Copperfields! (Even if he is a bit of a twat!).