The "positive" things about the issue/pull request etc. is presumably already explained by the author.
"Issue 1234: We need this feature because reasons A, B and C."
If you don't think that A, B and C justifies the change, send a downvote and/or explain why A, B and C aren't good enough reasons.
An upvote is sufficient to say that "Yes, I agree with A, B and C. Do it."
Of course, if you agree with the change but for reasons other than A, B or C, you can leave a comment to start a discussion. It has nothing to do with culture. It's just a way to minimize the redundant "Yes, I agree with what you just said".
"Issue 1234: We need this feature because reasons A, B and C."
If you don't think that A, B and C justifies the change, send a downvote and/or explain why A, B and C aren't good enough reasons.
An upvote is sufficient to say that "Yes, I agree with A, B and C. Do it."
Of course, if you agree with the change but for reasons other than A, B or C, you can leave a comment to start a discussion. It has nothing to do with culture. It's just a way to minimize the redundant "Yes, I agree with what you just said".