As a reader, you would visit the Constructive Criticism Card website, and tick off tickyboxes on a form. Upon submitting, you would have an HTML snippet -- a lovely image with alt text, linking to a page with customized links to assorted articles on writing (and doing it well).
The image and alt text bit of the card would read something like, "One of your readers thought your writing shows potential. They took the time to recommend these ways that you could improve this piece in future revisions, or take into consideration for future pieces. Please keep writing in good health." The picture would probably have flowers and ribbons on it.
This would help people who would like to give constructive criticism, but don't have the time (or, especially, the tact) to write out the things that this person could be doing better.
This would help writers learn.
This would help writers by guaranteeing that they have well-wishes along with the things that they could be doing better, and would hopefully come off as a gracious and loving gesture.
It would also help maintain boundaries for people who really don't want any sort of criticism, just love. The actual advice itself won't be staring them in the face when they read their reviews; the flowers and ribbons will. They can nerve themselves up to go read the things that they might be able to use some work on.