By the time someone aquires 500 points, they will likely have spent enough time on the site to learn the distinction between the two. While using text labels would be fine, I do not think that would be better than using the symbols.
The reasons for clicking may not be symmetrical, but the actions when clicking are symmetrical. The points go down with the down arrow and the points go up with the up arrow. This is symmetry even if the reason (agree, disagree, quality, spam, etc.) for the increase and decline of the points is asymmetrical.
The reasons for clicking may not be symmetrical, but the actions when clicking are symmetrical. The points go down with the down arrow and the points go up with the up arrow. This is symmetry even if the reason (agree, disagree, quality, spam, etc.) for the increase and decline of the points is asymmetrical.