The designers did a great job inspiring fear with this ship. First of all, the Enterprise is small next to it and it becomes clear it’s also a lot weaker. The very design of the cube (a square mass of twisted metal) gives one the impression that these cyborgs don’t have any remaining human characteristics, so how could they spare anyone? In general, the very mechanical nature of the cube is disturbing because as humans (in real life) we use technology more and more. It asks the question, will humans someday be part human/part machine?
Then, to make things even more disturbing, the Borg were bent on assimilating humanoids of all kinds, so it plays on a fear we humans have of still having a body, but being stripped of our personality and everything that makes us human. The Borg and Zombies have a lot in common.
1. The body lives on while the soul is questionable.
2. They want you to become one of them.
3. They come in large numbers.
4. Both are ugly.
Of course, the Borg are even worse because they're much more difficult to kill. Then there is their signature phrase “resistance is futile.” This just added to the suspense because there was no way there wasn’t going to be resistance.
Of course, there were many episodes about the Borg and their Cube, which just proves how well thought out the idea of the Borg turned out to be.
See the video for some Borg action.