Really they all should be fair game. In some ways actual critical feedback means more than the vague gushing comments you quoted, where they could apply to anything. At least you know the person has actually read it. It also takes a lot more time and effort to give actual critical feedback than a basic gushing wall of meaningless adoration. I completely agree with you, that isn't what flagging should be for.

Consider this an invitation, I genuinely want to improve, so if you ever think anything, don't hold back :)