Every problem Legend of Korra had, had a precursor in The Last Airbender. Ongoing romantic relationship had zero chemistry (I'm looking at you, Zuko and Mai). Despite linking elements to philosophy in the lore, gaining new ability was seldom linked to character development. Aang never really embraced poorly articulated philosophy on water, earth and fire, he just unlocked the abilities when plot time had come. And let's not forget avatar state, that Aang failed to unlock through mental discipline as guru taught him and instead unlocked it by means of acupuncture rock.

Character development for the lead? What's this? Aang started the story by running away from responsibility to North Pole, get my the middle story by running away from responsibility to Katara and resolved plot by running away from responsibility to a lionturtle. Let's not forget Lionturtle that conviniently showed up in the end so Aang could avoid taking a moral stand.

If not for enjoyable filler, you'd hate Aang too. Should it be condensed into a Korra-lenght seasons, it would be terrible.