One of the pleasures of visiting DC is in getting around via the Metrorail system. The underground stations are cavernous and seem to be everywhere; the trains themselves are spacious and incredibly quick. Everything about the system seems to be efficiently adapted to usage, as if they constantly tweak schedules and train allocation to meet the needs of commuters. Everything is clearly marked, and it doesn't take long before you can get all over town without even referring to the ubiquitous posted maps.
Perhaps more noteworthy than the system itself is the population that uses it. At every stop, regardless of crowd density, people step aside from the doors before boarding to let those on the the train exit. This has to be one of the most polite cities I'm familiar with.
I still remember visiting DC with my family as a child, when Metro made a very strong impression on me. It's likely that my desire for a carless commute today is rooted in that experience.
For a thorough guide to the system and the neighborhoods it serves, visit StationMasters.