Boomkin wastes a global cd/dps to apply Faerie Fire while shadow priest doesn't, useful on aoe fights (though, there are rarely many mobs that are 83lvl).
Shadow priests have Mind sear, AoE that follows the target priest is searing. Pretty useful for fights where mobs can/are moving (Sartharion, Thorim, Freya, Gunship battle, Lich King (vile spirits) etc).
Vampiric embrace stacks between shadow priests, having more than 1 in same party may eliminate need of healing for whole group on some fights.
Mind flay has 50% slow effect while channeling, also can be useful for some bosses.
Others have mentioned the rest already.