Personally, I'm really ticked off that they're doing this.
Mostly because I liked the partial ambiguity and "make your own canon-ness" of the ending of A:TLA. They didn't tie everything up, and a lot of the things they did tie up could easily change, because, come on guys, they're all TEENAGERS.
I would have loved an Avatar spinoff set in the past, maybe a few Avatars back, in the peacefull, pre-Fire Nation invasion world that Aang grew up in, and I love the idea of a female Avatar being the main character, but I'm really just not excited for Legend of Korra. So much of it just sounds like a self-indulgent futurefic. I thought Mike and Bryan were better than this. Though maybe, in retrospect, considering the ridiculous mess that was most of Sozin's Comet and The Last Airbender that they let happen, you really never can know.