1-They use Frost Magic( That the Mages use , and that can be easily incorporated into the Frost Mage spec allowing Frost Mages to carry heavy armor) , Dark Magic( Which Warlocks could use and that means that Warlocks can also raise the Dead) , Blood Magic ( Which Warlocks also use: Blood Horror) .
2- Death knights imbue the Weapons with Frost/Unholy Powers. Rogues can also imbue the weapons with Magic , the Shamans too.
3- Death Knights use Death Coil which is Similar to Mortal Coil.
4- the Frost Plague and Blood Plague can be easily incorporated into the Frost Mage spec and into the Affliction Spec.
5- Death pact is similar to Warlocks Sacrifical Pact.
6- They can dual wield---> Which Shamans , Rogues and Fury Warriors also do.
7- Runic Power is taken from the Imbued Blade and Shamans can imbue the Weapons too.
8- All their spells can be used by Warlocks and Mages.
9- All their Shadow Magic they use can also be used by Warlocks including Raise Dead.
Ironic Thread about the differences between Demon Hunters and Warlocks. The similarities i wrote above are the proof that Blizzard didnt care about the similarities between classes and that Blizzard created the Death Knight . The similarities didnt stop Blizzard from creating both Paladins and Priests. So guys any other ideas why Paladins and Death Knights must no exist ?!
Blizzard created new spells for the DKs and that can also be done also for the Demon Hunters.