Asami was fine in season 1 and season 4. But the writers forgot about her for the middle of the show.
It's kinda startling how Mako and Bolin failed so hard. Bolin's tears and dumbassery was so annoying as the show went on and Mako literally lost his personality. He was just "detective man" for parts of the show.
I think the rest of the cast more than make up for Bolin and Mako. They didn't bother me thaaaat much in the end.
Korra was very different from Aang, which was really cool and done well, IMO. Her being a waterbender but with a firebender temper was quite the concept. And her growth throughout the seasons was really, really well made.
The "old gang" appearances were very "toned down" and weren't "milked for fan service", which is also a positive. her appearances were good, and we saw, through other characters, that they were still "not perfect" characters.
And, as we discussed it before, I really love the villain cast, bar her uncle in season 2. With Amon's problem being the "plot twist".