I'll pop in here and say what I usually do about Thorn.
It shouldn't even be a Tank item. Tank gear should be about helping your team directly or indirectly. That can be with auras, with slows, with slow burn damage in an AoE, or even just a bunch of CDR to help you spam your CC more often.
Thron shouldn't be a Tank item. It should be a Diver item. It should help give tanky champions and edge over that Marksmen they just jumped on. To that end, allow me to suggest something completely different.
.
Grants armor, attack speed, 40 magic damage OnHit, and-
Unique passive: Your basic attacks apply stacks of Gored to enemy units, stacking up to eight times. Melee attacks apply two stacks per hit.
Unique passive: For each stack of Gored on an enemy unit, you reflect (percentage) of the physical damage they deal (pre-mitigation) as magic damage.
Clean, simple, and most importantly interactive. The problem with Thorn is that you don't have to do anything for it to mess someone up. That's why it must be so weak. But by adding in the requirement to actually interact with champions attacking you, suddenly there's room for counter-play. Don't let Irelia jump on you and you won't have your damage reflected. By adding OnHit damage into the mix, this new Thorn also helps to make you more of a threat even when you aren't being attacked, unlike live Thorn.
With the addition of Knight's Vow and Deadman's Plate to the game I would argue that there are ample armor options in the game for Vanguards, Juggernauts, and Wardens alike. Divers could really use some itemization tailored to them.