Nothing, except the fact that you're going to have Blood, Frost, and Unholy, which is the exact same spec configuration as the DK class. Yes, I know Necromancer fans don't believe that, but that's really the only way it works.
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That wasn't what we were arguing. The argument was school of magic. And yes, through the opening of Plague, DKs do have access to poison.
BTW, they also have access to poison because they're ALSO part of the Scourge which is the basis from which these fan-made Necromancer concepts are coming from.