I would upvote comments with true value (subjective of course) but my criteria would be:

• The comment needs to convince me that the person read the post well.

and

• I need to feel that the comment is adding new relevant information regarding the post.

or/and

• The comment is a well constructed criticism to my post

or/and

• The comment is an interesting reply to another comment.

or/and

• The comment is correcting something in the post or another comment.