Cultural Attractions/Nightlife
DC, quite simply, is a history buff's paradise. I can't count the number of museums of national significance -- and there are also a ton of interesting private museums (e.g., Holocaust Museum, Newseum). There are a lot of cool memorials and monuments, and some interesting sites of historical interest in the District and in neighboring states.
I don't know a ton yet about the DC nightlife, but my initial impression is that U Street, Dupont Circle, Adams Morgan and H Street are the major areas where bars, clubs and lounges are. I think this is easily one area in which DC pales in comparison to NYC. There are simply far more great restaurants/bars in NYC than in DC.