I think custom combat animations with a novel resource mechanic game and their strong visual identity is enough of a distinction. Ultimately, every class is only going to be melee, ranged, tanking, healing, or hybridizing those roles. The trick is in making it feel fun and new when you do those things. New rotations, new priority systems, novel flavor mechanics and animations, all of these things could distinguish a demon hunter from the other melee classes. It's really a matter of Blizzard choosing to embellish and flesh out a class we know almost nothing about.
It would be a damn shame to see them forever left as what could have been just because they were cannibalized on the whims of well meaning developers to give other classes some demon hunter homage mechanics, but it's possible that's just what they did. I think deathknights and monks both offer precedents to support demon hunters as well as to discourage them. I think people who insist they know one way or another have an axe to grind.
---------- Post added 2013-02-18 at 10:46 PM ----------
As if any race's hero we play aligns well with the deathknight theme? A demonhunter would be an antihero, just like a deathknight. They are the epitome of a night elf antihero.