Fair point! I definitely concede that I don't usually notice the icons (but maybe I might if they were a noticeably different colour to the rest of the page, e.g. red?).
Given that stickied threads do have that changed colour, perhaps it would be possible to implement an option for moderators to add the "quality post" attribute (+/- an icon, but with a different colour for that post à la what is currently the case with stickied threads, just without the 'pinned to the top of page' bit). I reckon this would solve any issue of visibility, but obviously the concern arises as to how it might actually look in the context of the rest of the forum.
Edit: This is the 'colour change' thing I'm talking about. Perhaps 'quality posts' might be a light green?
I'd have a go at mocking up what I'm talking about but I can't edit for shit