@Meddler: What are your thoughts on a "Frost Armor" ability ala Warcraft 3?

For the unfamiliar, Frost Armor was a spell used by the Lich that granted a target bonus armor and caused attackers to be slowed. Very handy against Blademasters.

While do this, they do so very weakly since they're passive effects. But what if we had something like this?

Shield target ally, absorbing up to X damage. While the shield holds, attackers have their attack speed reduced by Y% for a short time.

It'd certainly help make someone a lot more durable against ADCs, but only be marginally effective vs nukers/assassins/etc. Unique advantage? Yes.

Is this kind of skill something that you might add, or is there some broken part about it I'm not thinking of?