I couldn't agree more. I haven't been in three years or so; but when i was there even the crowds were polite. It's one of the reasons DC is so far the favorite city i have been to.
In the Natural History Museum i specifically remember looking at several large exhibits where people kept their kids from obstructing other patrons view for a long time. Adult crowds - looking at the Hope Diamond comes to mind - were smushed in for a close look but even then it was a polite crowd.
I'm going back there in September for a baseball game and hopefully one museum. I can't wait.