As I said upthread, it is mind blowing that this was aired on a Saturday morning in 1973. FOR CHILDREN.
It is surprisingly mature and unflinching. While the animation totally sucks (even by 1973 Saturday morning standards), it does manage to convey a lot of emotion. The voice talent does a remarkable job, considering there are quite a few "emotionless" characters.
Yesteryear provides a great deal of background on Spock and Vulcan that would fuel Star Trek for decades.
I would put Yesteryear alongside the best of any live action series.
The Animated Series was considered "Season 4" of the The Original Series by its creators and with veterans like DC Fontana on board, it's easy to see why.