On the surface, I should prefer The Legend of Korra to The Last Airbender. More adult, less heterogenized, more complicated, more urban, more intrigue. Unfortunately, I probably won't like The Legend of Korra nearly as much as I did The Last Airbender. I do respect it so much more.
It's rare seeing that much guts in what's ostensibly a spin-off. Not only does Korra have a set number of seasons, rather than following the typical American tradition of milking the cash cow as long as they fucking can, but they've got the fucking gonads to kill off (with one exception) every single character they lovingly developed over the three years of the series, radically adjust their expansive worldbuilding to the point of more or less starting from scratch, ramping up the emotional maturity to a point five years past the age of their original audience, plugging an improved art style on the thing and hoping for the best.
Korra is fundamental different, on pretty much every level, from Last Airbender and that's maybe what's coolest about it.
Unfortunately, from the two episodes I've seen, it does fall into a small handful of necessary flaws, things I wouldn't notice had I not the original series to compare to. In an effort to distance her from Aang, who was so non-standard for epic protagonists (fun-loving and carefree rather than angsty or headstrong), Korra ends up fulfilling the typical fantasy trope of 16-year-old destiny girl a little too well. On the same token, the Fire Ferret brothers feel a little like Diet Zuko and Sokka Zero, potentially competent characters down the line, but possessing less flavor and less drive than their previous-series counterparts.
I anticipate Korra will be great on the whole. It only suffers from the illusion of inferiority because of the excellence of its predecessor.