In the case of the Capitals I assumed that in DC Metro area, the Redskins, Ravens, Nationals, and maybe the Orioles and Wizards were more popular. Again, not because all of those teams were necessarily more exciting or better, but just sheer numbers of fans. Perhaps I'm just overestimating the gap between all of the big 3 and the NHL, but I just don't know. Both the Wizards and Orioles have been terrible for years, but I would be willing to bet money that if you polled 100 people in DC, you would find that more people had been to Wizards and Orioles games, could name more players on those rosters, owned jerseys, watched games, etc.
I've been to DC once, in the summer of 2002. I was 17 at the time. I hadn't really traveled much outside of the South at that point, so I'm not sure what my expectations of the city were based on. I just assumed that it was a really small area filled with government buildings and super rich neighborhoods where politicians and government workers past and present lived. When we visited the Capitol, we drove through what I seem to remember were some really run down neighborhoods with rough looking apartment buildings and just not a great area. I don't mean to insult those who live in DC, but it's just not at all what I thought it would be. There's definitely nice areas, but the juxtaposition of lifestyles is incredible; some areas are just night and day.
Dallas on the other hand has a really weird downtown, and now having been all over the country, I can tell you that it's unlike most major cities in the US. Comparatively, it's incredibly devoid of people moving around on foot, living there, etc. About the only thing to do in downtown Dallas is go to work. There's some neighborhoods in the surrounding areas, but the actual downtown area is not really known for much of a nightlife compared to similarly sized cities. I think a lot of that has to do with the fact that we don't really have an extensive bus/rail system that connects all the various areas of a pretty large metroplex efficiently enough to rely on that for commuting. You'll never have a problem finding an entire bench to yourself on the trains here. People here spread out further and further into the suburbs and just commute. For reference, I grew up here until I was 18, moved out to the Bay Area, CA and then came back after living there for a decade. San Francisco, Oakland, and San Jose are more like most major cities in the US. I suspect that Dallas is reaching the limit of expansion to where people won't want to make a 90 minute commute anymore and there will be the need for affordable housing to expand downtown.