In no way can Rust be considered "stable". The most recent version is 0.10 (that is, it's still very pre-1.0), it's undergoing significant breaking changes on an ongoing basis, and even its home page warns that "Rust is a work-in-progress and may do anything it likes up to and including eating your laundry."
Anyone who points out that Rust is just not stable yet is absolutely correct. Anyone who points out that it's looking unlikely that it will be stable before the promised end-of-2014 deadline for 1.0 is also correct.
It doesn't matter who points out these facts, or how many times they do so. The facts are still facts, and whoever expresses them is totally correct.
My suspicion is that the inappropriate downvoting is just immaturity on the part of the Rust developers. I realize that it can hurt to face valid criticism of one's work, but an important part of being a professional adult is to consider such criticism rationally, and to learn how to improve based upon it. Engaging in censorship of facts, however, completely contradicts with self-improvement.
It renders the voting system useless when it's incorrectly used to censor others, due to a lack of emotional control on the part of the person misusing the system.
It's one thing to downvote blatant commercial spam that has no inherent value. It's harmful to downvote perfectly legitimate commentary that you may just happen to disagree with, however.