Some more practical and less touristy things include:
1. Don't drink DC tap water (Northern Virginia water is usually safe)
2. It's fast paced. People speak quickly and have relatively small amounts of patience when it comes to waiting for service, in lines, on escalators as others mentioned, in traffic, for pedestrians to cross, etc.
3. There are a lot of big egos here, but don't take it personally. You'll especially encounter it when driving: people can't let others merge in front of them, there is tons of tailgating and, let's say "angry" passing, and overall lots of road rage here.
4. Speaking of driving, people completely forget how to drive when the weather changes (e.g. the sun comes out, it's cloudy, it rains, there's snow), which causes a ton of unusual traffic and accidents.
5. The whole area can't cope with even the slightest amount of snow, which is good for getting off of work/school!