The Last Airbender, for sure. Legend of Korra has no charm. I don't know if that's because its leads are late teens/young adults (who are by default less charming than kids) or because we never connect with Korra through the struggle of mastering the elements. Having her bend three out of four from the start takes away a big piece of the journey, both the physical, and the character's progression when learning what kind of person they need to be to bend. Then there's the supporting characters, or Team Avatar. Sokka was hilariously funny, while Bolin is the guy who tries too hard, Mako who is some strange Zuko-esque kind of bad boy, falls flat as he has no chemistry with anyone and appears to hoard Republic City's supply of hair gel for himself. Asami is... there. I mean, her fight scenes are beautifully choreographed, but character-wise, she's rarely gets past her face. Even at their worst, Toph and Katara were much more interesting than the current Team Avatar.
I also don't like the polar bear-dog mutant - it's something about not having properly done eyes (little black spots) that ticks me off.
Then there's the story... in The Last Airbender, we have the epic tale of saving the world from the Fire Nation who stands on the brink of complete and total victory. While in Legend of Korra, it's just set in a city. A boring, steampunk city where everyone and their grandmother can metal, lightning and bloodbend, terribly boring scenery (compare the city streets to say... the forest when Sokka and Katara got sick and Aang had to get frogs)... and cars. Freaking cars. burning_eyes
My absolute biggest gripe though, is how the finale was played out: Amon turns out be a bender (big shock -_-) and uses bloodbending to remove bending (I guess he shifts the flow of chi using the bloodvessels so that they can't bend) and Korra uses energybending to restore others' bending again (I was under the impression that energybending was less physical and more spiritual, going by what the Lion Turtle said)... and now she can also airbend, in the typical Loved-one-is-in-danger-and-new/hidden-power-is-awakened-scene.
Or maybe I don't like it because it's not very funny :/