Close vote seems rule-based, but the rule now can't be handled by a computer. What about up/down vote?
Both voting and close votes are opinion based, guided by rules
If you hover over the upvote button: you'll find some basic guidelines: "This question shows research effort; it is useful and clear". You'll find something similar for downvotes.
The same is true for close votes. When you close something, you specify the reason. However, if you look at all of the close reasons, all of them have elements of subjective, opinion-based elements. (e.g. Is this challenge similar enough to be a duplicate?)
There is a distinction however: As close votes matter far more, we have far more guidelines on their use, and who voted to close a challenge is public information.