Of course, disagreements are not insults and/or prejudiced against the author themselves. However, with disagreement without context, with no hint as to why the proposal was bad, there is little room for improvement. Furthermore, not everyone may have the same reason for disagreement. Some may feel the proposal is too vague, and has too few details. Others may feel it is too broad in scope and need to be more streamlined, i.e: a good proposal is in the core, but it is drowned by other things. Some may dislike the current syntax of the proposal, which the RFC-author could change for the better. One group thinks the proposal does not fit Rust, while another dislikes the core idea of the RFC. If the author is aware of why people dislike the proposal, then they may improve it.
I don’t believe a stream of up-votes and one of down-votes are symmetric. As an analogy, consider a novelist who has spent a few years writing a book they believe in - (sure, RFCs do not take years to write, but well a non-trivial amount of time) - on release, all they get is “this novel is crap” - and the novelist, whose self-esteem was not battle-hardened to begin with (a realistic scenario), never lifts the pen to write again. So, the effect of drive-by-downvoting (without given context) may be to discourage the author from writing some other RFC in the future (I may be re-iterating this point, but it’s a good point to re-iterate). People willing to do language-design, is not an inexhaustible resource, and we should treasure it. Accepting rejection in discourse is useful to learn, but not rejection without context.
I don’t, however, believe that having no time to explain why a proposal is bad is a valid reason to leave “-1” without comment. The author may have spent a great deal of time writing the proposal. At the very least, to respect their work, you should spend a little time explaining why you disagree or simply agreeing ("+1") with someone else’s comment that disagrees with context.
With respect to the illusion of universal agreement and statistics, I believe that, with a few up-voted comments of dissent at the top of the RFC-thread, it is still fairly easy to get a sense of whether or not the proposal has community approval or not. I don’t deny however that tallying may become more difficult, but I believe it is a price worth paying, while others may disagree.
To sum up: Disagreement is healthy and fine, but it should come with context.