You know how all those constructive criticism blogs fall flat on their face? Here's another one, inspired by a blog that came out recently that everyone knew was immediately going to fall apart again.
Here's a different take on it though. All you have to do is give ONE CRITICISM, no matter how big or small, to the person that's commented. And if you have no criticism for them, tell them you don't have a problem with them. The criticism can range from their behavior to how they write their stories to stuff that bothers you generally. If the OP chooses to, they can respond to the criticism that gathers up whether they agree or disagree. Everyone should be as polite as possible. Do not give out more than ONE CRITICISM.
To prevent redundancy, if someone beats you to mentioning a criticism in the comments below, I recommend either criticizing them about something else or saying your problems have been already been addressed or something.
I don't know how well this blog is going to work out, but... hey. Nothing wrong with trying something new.