You do need to understand, though, that when we tell you it's crowded, it's not a type of crowded you are used to seeing at DLR. NYE-level crowds are a whole different animal because large mobs of people tend to congregate together in certain spots, and it really makes it difficult to do things. Do you like to go to DLR and not be able to move around and do things? I don't know anyone who goes to DLR and doesn't like to be able to move around and do things!
While you may be thinking, "Yay! Lots of happy people having a great time on NYE! How fun," that may be true in some areas of DL - but not on Main Street. No one looked very happy in that crowd when we were there - and we were literally forming a human chain to push through the people on Main Street (trying to escape) who refused to move because they didn't want to give up their spot for midnight.
I'm not exaggerating when I tell you that the people on Main Street were smooshed together and it was at a claustrophobic level. It really was. Remember - my friend got so freaked out by it that she suddenly developed a "sore throat" so she could head back to the hotel for the rest of the night. I knew she had a tendency towards panic attacks, so I know that's what the source of the "sore throat" was. She's never done that before, in any of our DLR trips, no matter how crowded it has been - only on NYE. There was nothing fun about that particular part of it (on Main Street) - it was actually scary and no one looked pleased because everyone was uncomfortable.
You may be able to find some areas of DL or DCA that are not like that - that's entirely possible - but from what I've read/heard, the claustrophobic aspect of the crowds on Main Street on NYE has not changed in the years since I've been there. However, we did experience a blissfully uncrowded park for a few hours on New Year's Day - like up until about noon, and then it got busy. We knew that everyone was hung over and in bed, and we took advantage of that!