Korra sucks because the characters are crappy and unrelatable, the stories are short and isolated, the powers that used to be unique and interesting are trivialized and boring, there are big dumb robot suits that feel totally out of place, and there's barely any character progression. Also pro bending is gay and lame.
Avatar was great because you grow to care about the characters as they grow over 3 seasons from some dumb kids to formidable fighters. Aang coming to terms with being the avatar. Zuko's redemption. Sokka becoming a man. Toph just generally being awesome. Katara's growth and struggle with revenge. Building up to a climactic battle that is just amazing.
Aang vs the fire lord. Azula's descent into madness. And I particularly love Sokka and Toph's ragtag team to take out all the airships. Toph looking out of the airship and exclaiming how much fire there must be. Sokka having to throw away his unique sword in a desperate bid to survive. The whole thing is brilliant.