I wish it was easy. I wish we could just draw out a map and bam it's playable.
But that isn't how it works.
I have been making maps ever since wolfenstein
Wolfenstein - tedious but fairly easy. You only had to work with 2d and the textures were all the same. no lighting to worry with and build time of next to zero.
Duke Nukem 3d - easy to edit since you could walk around in the map as you edited. But the catch was that the engine was not true 3d, it was 2.5d. You could never have two floors exposed on the same side, so no windows on top floor with a door below. If you broke that rule you got HOM - Hall of Mirrors effect. I got around it by using sprites marked as impassable rotated 90 degrees and used for a floor. Textures/sounds easy, AI did its own thing.
Quake - First easy to edit 3d engine. first you laid out the map, then added the lights, then ran a program to compile what you could see , then you compiled the lighting, then pakd it all together, then you could load the map and see what it was like. There was no viewing of the map in 3d until you loaded it in game. So it was edit, compile, play. See something you want changed, edit, compile , play. Repeat. If your geometry was off just a fraction you got leaks in the level and you had to hunt them down till you found them. They could be anywhere and it could take hours to correct.
Half life/ source - good engine but the editor is not as good compared to editors like Unreal. It doesn't allow as much freedom in use of objects for the level structure which is why source engine games tend to be blocky using textures to hide problem areas and textures to make up the detail. The engine is more capable but the editor really needs work as it hasn't really been updated to keep up with current hardware.
Unreal 1 - fairly easy to use, had previews in viewport so you could get an idea of the level without compiling it. Textures and sounds were not hard to work with.
The current crop of game engines Unreal, Crytek, etc make it easy to see what the end result will be without having to constantly go into the game. But really that is where the easy part ends.
It would be great to just throw all sorts of detail in a level and make hundreds of detailed buildings, but you can't because the level would perform so poorly that nobody could play it. So another thing the level designer is responsible for is making sure it isn't too complex for the players computers. You have to use things like portals to hide details, clipping planes to block parts of a level off.
Include things like doors, moving platforms, vehicles. Unreal uses kismet and can also use python for the scripting, so you better learn one or the other.
http://upload.wikimedia.org/wi...6/Kismet_Roboblitz.PNG
Add on shaders. Shaders are the things that make the water look pretty and the slime look neon green. They require programming too. The editor helps , but its still something else to learn.
Lighting is way more advanced now, so radiosity is used more and more. Something else to learn though.
That is just the basics of editing for Unreal . Anyone can take a cube , hollow it out, throw some textures on it and say that editing is easy. But make some full size levels and you will find out how long it takes and how many different areas of knowledge you know have to know.
Very very few people actually complete a map, many give up along the way.