None of the SE sites I know of adjust their upvote/downvote/favorite and accept answer icons when you hover over them. I believe this could make things clearer.
Especially for the accept answer icon I believe this would help a lot. I find the distinction between the not accepted and accepted icon on Programmers.SE particularly bad. The outlines on StackOverflow indicate a clear difference.
I asked the question on UX.SE as well, as it is particularly relevant there. Perhaps they can add some scientific evidence.
Furthermore, I belief the icons should be at least consistent, which is also not the case on StackOverflow. Up and downvote doesn't have an outline, while favorite and accept answer does. Again, let's see what the UX.SE guys have to tell about it. ;)