1. Both are relatively small. Compared to the massive urban centers of New York, Los Angeles, Paris, and London, Washington D.C. and Brussels are practially villages, with respective populations of roughly 590,000 and 1,000,000. New York City alone houses more than 8 million people. When you count both captials' surrounding areas their populations increase, but Brussels and DC proper remain small in size and in number of habitants.
2. Both are government towns. Washington is home to the United States Federal Government while Belgium hosts the European Union. A large government presence sets the tone in both of the cities, with much of the population working in public service.
3. Both have excellent public transportation networks...sometimes. In DC it's a never-ending cycle of track maintenance, single-tracking, and frustratingly poor night and weekend service on Metro. In Brussels, it's bus routes that change without notice and tram drivers who will exit the tram when their shift has ended, even if it's in the middle of the route. Both cities are lucky to have what they have. Both sets of citizens have a right to turn complaining about public transportation into a local sport.
4. Both go overboard with security. It's impossible not to know the president is nearby when you're in DC. Whether he's visiting the Department of the Treasury or his favorite burger joint he is always flanked by a massive entourage of black SUVs, cops on motorcyles and sometimes even a circling helicopter. Big shot EU types also have escorted motorcades and the metro station that sits underneath the major EU institutions is often crawling with security guards.
5. Both get a bad rap. Many Europeans consider Brussels to be a dull, drab, and soulless bureaucratic town filled with dull, drab, and soulless bureaucratic people. Washington, D.C. is sometimes refered to as "Hollywood for ugly people." Ouch.
6. Both have no native inhabitants. Ok, that's not entirely true, but let's just say that as a result of the U.S. Federal Government, the EU, and NATO attracting workers from far and wide, you could spend a long time living in both cities before ever meeting a true Bruxellois or Washingtonian.
7. Both attract protesters. From dairy famers blocking the streets in Brussels with cows and tractors, to gay rights advocates marching in front of the White House, if there's a cause to be championed you can bet these capital cities will hear about it.