I agree with your intent that people need to moderate them selves when speaking poorly of a biz. I am not sure that post count is the best way, I have seen this suggested on other forums. One problem is someone with 1000 post is not any more intelligent/honest(not sure that is the right word) than some one with one. All posters started with one post at one point.
Def in these economic times we all need to cautious of our word, what effect one person effects all.
Maybe the poster should have to have been to the restaurant a few times before they can post, or know several people that have had bad experiences.
But I def like your train of thought!