The writers if Avatar are very bad at coming up with believable endings. Deus ex machina all over the place.
I thoroughly enjoyed the civil war aspect of Korra season 2, but the spirit stuff really brought it down. Also, the last two seasons have not matched the original series in terms of the stylish display of bending. For the most part bending is just a means of throwing projectiles. Unalock vs Tarlok was a good battle, but I can't think of anything else.
Korra as a character has shown no earned progress. Every time she does something beneficial it is because the plot demands it and not because her character was progressing to it.I don't even see how successive seasons can do much better. This season ended with two gods fighting. Anything after that might seem underwhelming in terms of scale.
If it weren't for good characters like Mako and Varrick this season would have been a complete letdown for me.