Lived in DC for 4 years, here's my thoughts:
Adams Morgan: The biggest bar and club scene in DC, a real pain in the ass to drive through on Fri/Sat nights, but lots of girls.
Dupont Circle: Good restaurants, some nice happy hour places, more laid back than Adams Morgan, not as many girls (it has a large gay and lesbian population).
Georgetown: Good restaurants, nightlife is a bit more frat-ish (best way I can describe it), decent girls, not really Metro accessible though.
Arlington: Some good happy hour spots and decent restaurants, definitely more yuppyish (crowdwise, not pricewise).
U Street: underrated, more of the cool-jazz-hip vibe, nice place to chill and just hang out.
Farragut North: Sucks.
Cleveland Park: Some decent restaurants and bars located along a strip. Recommend checking Ireland's 4 Fields for a good Irish pub.
For restaurants:
Dupont Circle
Georgetown
Bethesda
Arlington
For clubs/bars:
Adams Morgan
Dupont Circle
Georgetown
Arlington
For music/lounges:
U Street
Eastern Market (good jazz clubs)
Yeah, DC is pretty damn humid in the summer too.
In terms of people, the good thing about the city is that its pretty diverse (from an international standpoint).
In terms of socio-economic diversity, I was pretty shocked (coming from a New Englander's perspective). When Dave Chappelle said that statistically there isn't such a thing as a poor white DC native, he really wasn't kidding. Also, you can't go to a party without having some bring up politics. Happens every time. I swear. Finally, the first thing someone usually asks you is "what do you do?" -- just an interesting insight.