http://schema.org/downvoteCount
http://schema.org/upvoteCount
http://schema.org/Comment

Via JasonJ: re https://schema.org/Comment "the property descriptions for downvoteCount and upvoteCount are specific to Question but the properties apparently apply to comments as well."

Also maybe "The parent of a question, answer or item in general." -> "The parent of a question, answer or comment item in general."? (http://schema.org/parentItem)