Looking at the clip, the animation quality seems so-so if you're looking at the characters in the background. They're just static.
I've been going through this in my head for a couple of days now, and I've noticed two things:
1) The animation still looks great, BUT, you're right, some background characters seem more static. Although,
2) There's A LOT more characters than in Book 1, both recurring and background, in each episodes. There's more citizens in Republic City, Zaofu, Ba Sing Se, armies, guards, and airbenders. I remember "Turning of the Tides" in Book 1, when the Equalist attack Republic City, and the I couldn't take my eyes off the streets and how empty they were. Amon was bombing the whole place, and there weren't even any cops or ambulances, or screams for that matter.
I notices they set most of the scenes in Republic City either at night or very early in the morning, probably to avoid having to animate people doing normal stuff or running around.
Now, in all three later Books, scenes are set up so that the amount of people we see on screen is believable in accordance with the situation, or more people are animated as background characters, but not as well done as in Book 1.