I think it's pretty great.
Maybe I'm just bad at being critical, but so far there's almost nothing about this show I don't love. Just like in ATLA the characters are interesting and engaging, the art is excellent, and the emotional pitch manages to run from exciting to comic to earnest without a misstep. The Legend of Korra definitely seems like it's headed towards a slightly different, darker tone than ATLA, but I'm going to give it some time to see whether this is well-handled or not. So far some slightly more mature themes have popped up, some disturbing things have been hinted at—not that ATLA hasn't already gone into some mature territory—and I like the way that's the show is building up a more complex world than the one we've seen before. There isn't just a bad guy that Team Avatar needs to defeat. There are the Equalists, the triads, the police, the White Lotus, and the people caught in the middle, and it seems like the show intends to examine the fact that, although some of these groups have opposing ideals, it doesn't mean that they're necessarily wrong.
As for the setting: good world building is something that appeals to me personally, so I appreciate the details like seeing lightning bending used as a power source that give the city more verisimilitude. I also think the steampunk-y details just look cool: who doesn't appreciate a zeppelin?
Yes, I had hoped that if we ever got a sequal to Avatar it would focus on the Gaang, not someone new. I'm disappointed that we don't get to see Ursa or the aftermath of the war and that most of the characters from the original show are gone, as I think most fans of ATLA are. But I think it's completely unfair to be disappointed with Korra for what it isn't
—judge it for what it is.
[Written after watching the first three episodes.]