Star Trek: Voyager introduced the Kazon, a race considered so "unremarkable" by the Borg that they were rejected for assimilation. One of the most feared races in the galaxy, the Borg have invaded countless worlds and absorbed so many different civilizations into their hive mind. The Borg seek perfection, and so add the biological and cultural distinctiveness of other races into their own in the hopes of achieving this. Surprisingly, though, there is one race the Borg have rejected; Species 329, the Kazon.
According to Seven of Nine, there's a simple reason for this; the Borg encountered a Kazon colony, but "their biological and technological distinctiveness was unremarkable." It seems the Borg considered the Kazon to be "unworthy of assimilation." In their view, the Kazon would not add anything to the Borg Collective; rather, they would detract from it, moving the Borg further away from their goal of attaining perfection. It's a pretty harsh assessment, but hilariously when he heard it, Seven of Nine's fellow Voyager colleague Neelix - far more familiar with the Kazon - couldn't reject it. "Good point," he replied.
The Borg Vision Of Perfection Rejects The Kazon
It's never been entirely clear what the Borg mean when they say they seek "perfection." Most viewers have assumed the term refers to the entire universe being brought into harmony as part of the Borg Collective, and that perfection will be achieved when all beings are as one; that does not seem to be the case, however, because in that case the Borg would not reject the Kazon. Rather, it seems likely that the Borg imagine perfection to be something intrinsic to their own being - the ultimate "evolution" of the Borg themselves. This presumably means the Borg assimilate races with biological, cultural, and technological elements they believe they do not already possess. The Kazon were irrelevant, simply because they offered absolutely nothing new.
What Do The Borg Believe Perfection To Look Like?
Star Trek has always been about discovery and exploration, and there's an odd sense in which the Borg quest for perfection focuses in on those themes as well. The Borg believe themselves to be lacking, yet to attain perfection in some way, and they seek to incorporate new elements into their Collective to help them move closer to their goal. Presumably they would only be content when they had explored all the galaxy, absorbing all the positive traits and technologies they encountered, leaving only the few races - like the Kazon - who had nothing left to offer. The Borg would then reach a plateau, because they assimilate rather than innovate; with nothing left worth absorbing, they would no longer be able to change and grow.
Star Trek's Borg Queen once opined that perfection is found in "disembodiment," so presumably the ultimate goal of the Borg also involves becoming a Collective that transcends the physical plane. Should the Borg ever accomplish this, they would no longer be a physical force, but a psychic one; their Collective could therefore expand without ever being seen, and nobody would ever be able to oppose them. Fortunately, as great as the Borg empire may have been in Star Trek: Voyager, it's clear they will never accomplish this goal.