A reference article covering a few small pointers on creating a suitably authentic real world setting. Bear in mind its a little out of date with the release of next gen games like HL2 and Farcry.

Locations
The most important thing to remember about real world settings is that everyone who plays your maps will more than likely recognize the setting, whether it is a subway or warehouse. Thus you will have to make it look as much like the real life setting as possible. Look around or get photos of these locations for inspiration.

Exaggerate The Realism
Take the first level of Soldier Of Fortune where you have to jump on to the top of a subway train. Not particularly an every day occurrence but still possible. Or what about Max Payne where the building sets on fire and you have to run out? Think about action scenes in movies. If it just involves walking down some streets then it�s not going to be exciting for the player.

Scaling
Scaling is much more important in real world settings. For instance on an alien planet you could get away with having doorways that are massive, but in a real world map it would not fit. Bad scaling is one of the main things that spoil the illusion.

Detail
With real world maps there can be the risk of you putting in too much detail, such as door handles. Avoid things like that, as current games cannot really manage all those little details and you will get a headache from working at such a small scale. It will rarely come out looking right.

When I mention about recreating locations, I mean capture the most recognizable features and structural shapes of the area. For instance, a subway has advertisements, long stairways, curved tunnels for the trains etc. In a game the player won’t notice other details. He will notice those general aspects that define it as a subway.