Meh, deathcap is a kinda "over powered" item, i personally think it gives casters too much of an edge.
Casters needed Deathcap. Ability Power only scales multiplicatively with the deathcap effect, magic penetration and cooldown reduction. Cooldown reduction is very difficult to get on a caster, seeing as the only two real caster items with cooldown reduction are Deathfire Grasp and the Ionian boots, which are often inferior to get over Sorcerer's Shoes. Leaving the only commonly viable choice magic penetration.
On the other hand lets count the ways attack damage scales multiplicatively!
1. Critical Chance
2. Critical Damage
3. Attack Speed
4. Armor Penetration
5. Cooldown reduction (on champions with abilities that scale with attack damage)
Now look at those stats. They are INCREDIBLY easy to get on a carry. Critical chance is built into many core items, critical damage is built into the quintessential carry item that almost every ranged carry gets. Attack speed multiplies the effects of large amounts of attack damage much more heavily than cooldown reduction does, and is often attached to many other effects which greatly increase a carry's usefulness. (More damage on Last Whisper, Damage and armor reduction on Black Cleaver, Lifesteal and armor reduction on Stark's Fervor, Magic damage on Madred's Bloodrazors and Sword of the Divine, Critical chance on Phantom Dancer). Armor penetration is like magic penetration, but more plentiful, off the top of my head five items offer some form of armor reduction, seven if you count items that add magical damage to your attacks, helping to avoid people's main form of damage resistance against you. On the other hand casters have four items offering magic penetration, and no real way to add physical damage to their casts (unless it is built into the character). Cooldown reduction is offered on Youmuu's ghostblade which also offers temporary attack speed, critical chance, armor penetration, and damage.
Casters needed their own Infinity Edge. It isn't overpowered, it was a necessary item that was missing. I would go even farther and say that casters do need more items and versatility in their builds.