As a whole, I found season 1 to be an wonderful start to the series, with an intriguing plot line, nicely-realized characters, and excellent production value.
Without revealing too much, I will say that the main plot manages to tie together a variety of threads and themes in a very clever way, and the central "mystery" is beautifully handled: the solution is surprising, but not completely out of nowhere and still satisfactorily solvable to the very watchful viewer.
Korra herself is one of the most likable teenage protagonists I've seen in a long time. Talented, funny, good-hearted, but also deeply flawed and human. And kudos to her voice actress, who makes her even more likable with her consistently fantastic delivery.
It's also wonderful to see a kid's show where the adults are so interesting and well-developed. Tenzin in particular is refreshing as an archetypal "wise old mentor" who also happens to be a goofy family man.
As per Avatar standards, the action set pieces are thrilling to watch and gorgeous to look at, particularly in 106, 108 and the finale. The show makes excellent use of combining ancient bending practices and customs with modern styles and technology, giving a very "real" feel to the world.
My only minor qualm with season 1 is that the pacing feels a little off here and there. Though interesting in its essential dynamics, the love triangle bogged down the plot at times simply by taking up too much screen time. But overall there is very little I would change about this season. Can't wait for more.