That, in effect, sets up a parallel karma system. There is the normal karma, visible and both up- and downvotable. And then there is the karma of dismissal which is unseen and can only go down but never can go up.
Besides that, the system implies personal "hide-this" flags for all comments. The Dismiss button, then, does two things simultaneously: sets the hide-this flag for the comment and decreases the comment's dismissal karma.
Besides that, the system implies personal "hide-this" flags for all comments.
That would be the little minimise button in the corner.
If it's worth saying, but not worth its own post (even in Discussion), then it goes here.