It is proper Netiquette to provide constructive criticism. Don't leave negative comments without suggestions.
Constructive criticism to help users learn from mistakes.
Use the sandwich technique to post constructive criticism on the internet. Sandwich criticism between compliments. Begin a comment with a compliment. Tell the user what you would like changed. Then leave another compliment. This way the person receiving the criticism does not even notice it because they focus on the compliments and take not of the criticism.
Provide constructive criticism because we may not know how to improve without it.