It appears it's only available to those that are green or red nicks...
they can give positive rep to a post or negative rep. They can also add comments to a post.
For example the initial post has received mainly positive reputation, with one person giving it a negative rep with 'homo' comment...
That negative rep vote took me down from "Borked is just a nice guy" to "A jewel in the rough" when you hover over the green bars (and also removed 40 or so rep points).
Pretty useless to have rep points though if you can't use them, so dunno why the feature doesn't become available to those with positive rep, even if they aren't a green nick