Just about everything past mid-first season is gold. The Ba Sing Se episodes ARE great, but they're not the only high points in the entire series. And even their more narrative driven arc allowed for things like "Tales of Ba Sing Se", which didn't advance the plot but was still great.
I re-watched the series before Korra aired, and ended up liking the third season stuff a lot more than the first time I saw it. It keeps just the right amount of "strangers in foreign territory" feel with the lightheartedness of kids saving the world. Then again, it does have that Painted Lady episode.
Korra is it's own show, though, and intentionally sets out on a different path from Avatar. The characters are older, the animation tighter, the themes more mature, and the plotting is much more laser focused. The downside, though, is that Korra neglects a lot of character building in its side characters. Characters like Asami and Bolin are great, but still kind of shallow, and Mako isn't developed at all beyond his basic role in the plot. And by the end of the first season it kind of goes off the rails and forgets what it's doing.
I don't mean to rag on it, it's still pretty great, and with more seasons to come definitely has a chance to flesh out the series. It's just difficult to give it much of a pass when the first season was intended to be the complete series and doesn't develop very well.
