It's pretty amazing in arena... counterspell a holy priest now he can't cast anything from the holy tree for 10 seconds. Oh and I don't know if it is a bug, or intended, but if you counterspell a priest and he dies during the timer, his spirit of redemption will sitll be counterspelled. Counterspelling the priest right before he dies (if he was casting) makes spirit of redemption a useless ability.
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As of 2.0.6; "Counterspell no longer triggers the global cooldown."
Monsters don't believe in spell schools. When you interrupt a monster, you only put that specific spell on cooldown. For example, Counterspell Holy Smite, and the monster might just heal itself the next second. (yes, with a holy spell)
edit: while on the PvE part of this spell, the threat mentioned in the tooltip is abysmally low.
Tooltip
Counters the enemy's spellcast, preventing any spell from that school of magic from being cast for 7 sec and silencing the target.
Spell facts
Datamined from the current retail build. Numbers that scale with level or stats are shown at their base value.
More Mage spells
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