Demon Hunter is a melee class, thereby, they do not fit with almost all of Warlock's spells. (MELEE CHAOS BOLT LOL)
Warlocks enslaves demons to fight, Demon hunters do not.
Demon Hunter is able to manaburn, warlock is not, and it would be the only melee class that would be able to manaburn, could easily lead to interesting combat mechanics.
Demon hunter are able to infuse life in shadows and felfire, and makes it swarm by parasiting ennemies, warlocks aren't able to. (C.f Illidan, his felfire elementals and his shadowfiends)
Demon hunter can feed on their opponent's soul (C.f Illidan), warlocks can uses ennemies soul to cast certain spells but they can't feed on it. We could say that warlocks are using their opponent's soul to heal themselves while life draining but nothing shows that.
Warlock's immolate is a mono-target dot while demon hunter's immolate is an aura (Bringing the activate/desactivate your aura whenever you wants concept which could be interesting).
Demon hunter uses fire, felfire, and shadow magics, only common points with warlocks, but certainly not with the Deathknights which are nearly the opposite.