Legend of Korra appeals to be way more mature than its prequel: The Last Airbender. Whilst that gives an edge to Korra, it also removes that childish and innocent humor enveloped in rather serious conflict.
Still, do not be fooled - Legend of Korra is an impressive animation work; it goes flawlessly in graphic and motion design, it doesn't lack a humor nor a good, intriguing story. Korra is basically everything Aang wasn't: She's stubborn, hasty, inpatient and not as shy as Aang. I think they made quite a good move here since we already have three season with over twenty episodes to watch a mindful, calm and shy Aang. The gathering, or gang if you wish, are also kind a different. There's that innocent, teen "Love squared" as I like to call it which is more amusing than serious, though. Then there's again one "wisdomous" guy with only patience and non-conflict solution on his mind. There are also many others; lots and lots of different characters that you will surely enjoy in. It's actually still early to judge the show because it unravels itself episode by episode. As the more episodes are aired, the more you realize about the story of Republic City and all its mysteries.
To short the long talk, Legend of Korra is most likely more mature than the Last Airbender (which actually brings another question: if Tenzin is the Aang's only child (or so I understood), then there's only four (upcoming fifth) remaining Airbenders - We'll see), with not so much a childish humor (which is still there, don't worry) and a story which untangles really slowly and forces you to watch the show from episode to episode. If you loved the prequel, you'll love its sequel.
