For me it was the villains. I think LoK had some of the most memorable villains to date, even they received some good development.
Yeah i think that's the switch between the two series.
The last airbender had more memorable main cast but the villains (bar azula) was run of the mill
Legend of korra on the other hand had amazing villains, and a average(mako) to above average(korra asami bolin) main cast.
Both shows get high marks from me, and the topics they tackle without giving a cookie cutter resolution is pretty amazing compared to the typical ("we are nakama" i see in shounen)