Why it's the Second Greatest ST series after TOS
1) They got all the original actors to voice-over their characters.
2) Even though 70s Saturday morning animation was limited by today's standards, they did a good job.
3) The stories used the freedom of animation to stretch the boundaries of what could be told onscreen. It wasn't wimpy. For example, the very first episode features the Enterprise encountering a 300-million year old spaceship which was destroyed under mysterious circumstances. A very spooky ep!
4) It introduced really cool sci-fi effects. It is this series which introduced the holodeck, which appeared again in TNG. It also had a really cool other effect: the "Life Belt," which allowed you to go to a vacuum environment and have this yellow force field around you which allowed you to live.
5) The music was different from TOS and good. Every other ST series since the original has just sucked, music-wise, but not this one: the theme song was exciting, the incidental music was appropriate: when there was mysteriousness going on, the music was spooky; when there was battle happening, the music was military.
6) The reason TOS has survived to this day is simple: it had a quality of human pathos, that is, the stories had a quality which transcended being merely stories and spoke to the universal human condition which we all experience today. This series did as well, and this is the single most important reason why it is the second greatest series in the ST next to TOS, the greatest SF series to date.