There are lots of differences between the two classes, and in fact, I’d be tempted to say that the only things they really have in common are the name and the ability to heal. Starting out in leather, and graduating to chain armor, the Aion Priest is a much sturdier class than WoW’s Priest which never gets out of her cloth gear. Additionally, Aion’s Priest is encouraged to melee. In WoW, we like to call that ‘stick-whacking,’ and it’s largely frowned upon by the better class of players.
Even though Aion’s atmosphere is much less cartoon-y and fun that that of WoW, there is nothing there to compete with the outright evil-gasm a Shadow Priest gets from phasing into her purple form and stacking Shadow Word: Pain on everyone in sight, then Screaming them all to the four corners where they die slowly, in agony. And that, my friends, is the way Priesting was meant to be.