I remember in the Avatar State episode Roku telling Aang that the glow is the past Avatars giving him their power and wisdom. Kyoshi's eyes glowed in Avatar Day, and Roku's eyes glowed in Winter Solstice. Airbenders used those tatoos to show they were Masters. Water tribe people if i remember correctly wear a special coat? I forgot about the Fire Nation and Earth Kingdom. Either way, the Airbenders are the only Masters whose sign is physically on their skin. So, maybe since the tatoo is a physical part of him and since the Avatar's spirit when activated takes control of his whole body, that is why they glow. A close stretch, but maybe it will open up new discussion!