there is a large lack of character development and lots of the scenes are painfully cringey to watch. also, many characters seem to just have "inborn" abilities that are better than those of people who have trained for years (for example, jinora vs tenzin) which is just annoying. also, many of the villains are aware they are evil and just don't care, which is uninteresting, unrealistic, and makes many of the villains just boring to watch. many characters also seem to get the short end of the stick, every single time. (for example, tenzin always seems to do everything wrong even though he tries so hard. he is never rewarded and it genuinely made me frustrated many times watching the series.) also, korra is stupidly stubborn and unappreciative the entire series. she never really grows out of it. many characters repeat past mistakes they have made, even after they have had encounters that should have taught them to never do it again, like korra's stubborness.
however, there are many good aspects as well. the romance is sweet but also realistic with fights and breakups. there are also many side stories and flashbacks that are incredibly enjoyable and the drama makes sense. the vast and unique personalities of each characters add comedy, drama, and anxiety which keeps you watching.
i found that the flaws of the show frustrated me more than the pros made me happy. im glad i watched the legend of korra, but there were many, many episodes i had to force myself through.