I like that Korra has had 13 episode arcs instead of the 20 episode arcs in ATLA, but I think that the writers change course as they receive fan feedback. Season 1 wasted a lot of time on pro-bending matches and taking the bending away from people. It was boring. Season 2 was an improvement, but it still lacked a lot of the impressive bending seen in ATLA. Season 3 addresses the lack of airbending and has been the most impressive in terms of bending form returning as the feature of the show. If the writers hadn't wasted so much time on conflicts which take the show away from what made ATLA impressive there would have been more time to focus on the spirits. Although I dislike Zaheer's motive so far I still enjoy seeing highly skilled benders fighting again.
I do wish the spirits were more of a focus or at least had more of a significant change to the world, but perhaps that plotline was saved for season 4.
I also agree that characters like Asami and Bolin have less to do as the plot progresses, but that is the case with many plots. Bolin is comedy relief to lighten the mood and Asami is the resource person. Without her help the group wouldn't have much money, transportation, or technology. It's better than relationship sub-plots.