what those two terms ["noise" and "signal"] mean varies from person to person and may in fact have wildly different meaning to the people voting up or down, but that seems to be the intended effect of the voting arrows

It's probably the ability to aggregate these wildly different meanings in an interesting way that makes the voting system good.

Sometimes just saying "no/yes, I (dis)agree with you" is sufficient and the site is better served by encapsulating these statements into up and down votes rather than encouraging the equivalent of "+1 I agree" messages

That's exactly right.

In this whole area, less is more. Many of the objections and suggestions for changing it strike me as overengineering. Given that the site is the creation of an inveterate minimalist, though, I doubt we have too much to worry about. :)