Have minor in-perfections on environment/weapons which were caused by normal day-to-day work.
eg:
- a table top has not got a perfect wood skin.
- metal is very often scrached (take Crysis weapons for example)
- edges are always slightly round (look around you)
Another thing:
Have rotating/scrolling message boards giving small Black Mesa background stories.
eg:
- “Staff meeting postponded.”
- “Last accident: 21 days ago”
- [the time]
- “A good beginning makes a good end.”
- “Do a little more each day than you think you possibly can. -Lowell Thomas”
- [1 in 20 chance of Spoiler]
- “Do every act of your life as if it were your last. -Marcus Aurelius”