All other clases have some sort of AoE damage. Seed does AoE damage and applies corruption without it being talented. Destruction has (altho it does low damage) Rain of Fire. Same goes for Starfall, you can make it more powerful by specing into specific talents that boost its damage.
What do Shadow Priests have? Nothing beside mind sear which damage stops as soon as the target you're channeling mind flay on dies.
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Can you enlighten me please? And I'm not being sarcastic.