Demon Hunters tend to have their features changed over time as they manipulate the dark energies, transforming slowly into something very demonicm From wowpedia and the RPG book (Guess Blizz confirms the RPG book information on this matter with that picture):

"As a demon hunter grows in power, they undergo a gradual evolution, becoming more and more demonic in appearance. The changes are cosmetic and can take many different forms, from burning eyes to black blood. By the time the demon hunter reaches the pinnacle of his development, he is a twisted and dark version of his former self - though the chaotic energy in his body does not affect his personality and allegiance (or so the demon hunters claim), it wreaks great changes on his physical form. The demon hunter’s gradual evolution achieves its pinnacle as they become a demonic entity. Their appearance has changed to something frightening: The exact changes vary, but can include glowing red eyes; horns; clawed feet and hands; nonfunctioning wings; cloven hooves; a spiked tail; dark skin; a deep, rumbling voice; and similar cosmetic changes. The demon hunter becomes an outsider."