Comment 10 for bug 686133

BEcause the minus sign (−) is centered in the button and not placed at the bottom, it does not convey "minimizing down". The minus is placed in a neutral position, so it is still valid no matter where your launcher is, at the left or at the bottom.

If the icon were a vertical line, I would think that the button is for semi-maximizing, or something more confusing. Anyway, these are symbolic, metaphoric icons, I don't believe that they must represent the exact desktop. And I don't know about anybody who got confused by the minimize icon, all of my friends clicked the minus expecting to minimize the window, and that ocurred.