We used to have a rating system, but people would slam stories out of spite or squick, so it was abandoned when Miss K. remodeled the site.
Flames are not allowed on Dokuga, so anyone receiving ANY kind of flame should report it to an Admin. (Miss Kagura, Abraxas, Nobody).
"Your story sucks.", while not exactly a flame, is not considered constructive criticism either, and the reviewer would most likely receive a warning.
Authors commonly visit the profile page of a reviewer and leave their thanks under the 'comment' tab. You can access the profile page of a reviewer easily by clicking it if it appears in red on the review page.
You can most certainly raise their karma if they have posted anything in forum and you find their post, I'm sure they'd LOVE it!