User blog comment:JBox2D/A dedicated anti-mage champion/@comment-4714821-20140223020109/@comment-7709681-20140224201054

W - Even if is it has a static cooldown the spell steal is a powerful ability, enough to be an ultimate, increase the cooldown of the ability or decreasing the time in which you can hold the denied spell won't fix it.

E - The highest damage reduction coming from abilities is 50%, that's why is a problem. About the passive, this would be acceptable if it where an active, but is not, perhaps you'll want to only deal bonus damage on-hit equal to a percentage of the target's AP, but passively stealing AP is bad.

R - When its said that is "counter-productive" is because the second damage component of the ability wouldn't apply frequently enough due to the silence, even if its short, besides you already have Q's silence, which later-game becomes spammable.