I would just like to vote for the current system. While negative can be hard, It is useful if it's tactful. I am learning a lot about keeping my audience in mind with this, and I might not think about that so hard with a different system. It's good for me. I don't even care so much about constructive (though of Course I would prefer that) as long as it is tactful. I think there are too many variables in the other suggestions.