It suggests that the piece was effective in some way, although may not specifically say how it was effective or suggest how it could be made even more effective. Even if just one person does it, the load from the list can double or triple causing many people who wish to participate to leave. It may seem impersonal to send brief encouraging statements to multiple people into one e-mail, so it may be better to just send those directly to the author anyway. The difference between a critique and a comment is that the critique moves one step further and gives the author something concrete that they can work with. Comments can certainly encourage or promote fruitful discussion, but a critique gets to the bones and suggests a next step for the author.
Holistic evaluations are likely to be the most prevalent on the list.
When you look at a piece holistically, you are gauging the impact of the piece as a whole. You point out its weaknesses and strengths, and help the author to find trouble spots. Your dialogue is very believable, sounds natural if you read it aloud. More details about setting may make this clearer to the audience when the change takes place. These are general suggestions to the author, something they can work with.