Downvotes have the effect of suppressing the content expressed, making it less likely that it will be seen by others.

I'm not proposing a specific guideline for their use. I'm opening this up for conversation. I'll outline what I believe are a few reasons people may downvote a comment. I do not downvote all comments that fall under these categories, or necessarily regard all of them as valid downvote reasons. I simply present them as a starting point because I honestly think this can be an interesting and productive conversation to have.

1) Inflammatory/personal attack

2) Off topic

3) Factually incorrect

4) Perhaps factually correct, but incomplete or poorly reasoned

5) An opinion you disagree with

6) An opinion you disagree with that is also not well reasoned

7) An opinion that is in some way not productive to the conversation

8) Short comments of the sort "Yes, I agree"

9) Factual claims that may not be general knowledge, and are offered without citations

10) Comments that demonstrate the author has not read the posted material

11) Comments with significant misspelling/grammatical errors

12) Others?