World of Warcraft if they're not using abilities? There's not much of one. Which seems a little odd, seeing as these are two classes with immense difference in lore. That's why the art team is showing off a preview of things to come in the form of cosmetic class customization. The idea is that classes should have bits of visual flair to distinguish them without being tied to stats.
The preview shows off potential accessories for Rogues, Hunters, and Paladins, accessories that start off simple and get more elaborate over time. The design blog also points out that this is not something likely to be in for launch of Warlords of Draenor, although whether it might be included in a later patch or the next expansion altogether is somewhat ambiguous. Check out the full design preview for more details about what the designers envision.