The fix is simple: use a different symbol. Something that more precisely, objectively signifies "not simple and not nasty". This will create a more permissive gate that allows opinions of moderate substance and moderate tone through (an "err on the side of allow" policy). Inevitably the symbol must also consider the tendency of moderators, being active participants in a discussion themselves, to favor their own views.
Taken all into account, I suggest the "endorse" link be renamed "tolerable". (Rather than an action, it becomes a kind of flag.) This has the explicit connotation of erring toward permissiveness of differing views, and contrasts the two fears expressed in the post: comments that are boring or unprofessional/spiteful/mean.
(And as it stands, upvoting already signifies "endorsement".)