I have to admit I have a love/hate relationship with the detailing phase of my maps.
When I have a clear idea of the area I am creating its fine, its fun too.
When I don't have a clear idea on the area it can be frustrating and so so dull.
Bearing in mind I'm building all my areas with gameplay in mind primarily, I often dont have a clear idea of the type of real world space it may become. Sometimes the two come together seamlessly but more often than not I find I have to work quite hard to get my areas to look believable.
Basically, I think the goal when detailing is to add just enough detail that the player accepts the reality I'm presented to them. Sure I could go way overboard and add every concieveable detail however I'm sure that players would prefer me to create more playable areas of a map than adding a working telephone to an office.
There is no hard and fast rule about how much detail is enough but I want you to try an exercise for me.
Pick a section of the HL2 game or its episodes that you remember really well. Load it up using the console.
(for example "map ep2_outland01a"). Now punch in the "sv_cheats 1" "noclip" and "notarget" commands.
Now you can fly about the level at will and get a really good look at how Valve put the map together.
Look at the detail and lighting especially.
Normally, I find during this process that the elements of the map that I thought were particularly complex are actually fairly basic. By adding some of these basic detailing touches to your map you can give the appearance of detail without spending a huge amount of time on it.
A good example is a small balchony on the side of a building.
When I looked closely at these in HL2:EP1 I realised that actually theyre simply made out of two brushes. One to form the floor covered in a concrete texture, and a second brush that forms the railings. This is just a square brush with a railing texture applied to three sides of it.
Simple to make, but adds a great level of detail to the side of a building.
Lighting can do a great job of masking an under-detailed map. Long shadows especially give off the illusion of a detailed environment when actually detail has been kept to a minimum.
Some nights I'm not up to mapping or detailing. My brain just can't take it. Its weary... so I tend to spend these evenings creating stand alone detail prefabs.
Little things like flaming torches, working cupboards with items inside, fire escapes, window boxes, non-working doors with locked sound effects. These are things I can drop into any map to flesh out the world.
By building up a huge library of these items, detailing your maps can become far less painful. Feel free to share them online too. These kind of details can bring everyone maps up in quality.
Testing the detail
Players will always tell you a map needs more detail so be careful of this. Don't clutter you're map too much. Also, you must be sure that whatever detail you add, make sure it doesn't affect the gameplay.
The player getting snagged on a light model during a difficult jump puzzle can really spoil a map.
My rule for detail models is: if in doubt, make it non-solid.
The player will accept passing through small models. You might want to consider converting any small detail brushes you have to func_brush and making them non-solid too.