If you're a marksman and a tank builds Thornmail you are pretty much fucked for the rest of the game. Not only does it hurt you when you attack them, it applies grevious wounds so you can't counter it with lifesteal. It is such a strong item that goes under the radar for no good reason.
Thornmail's powerful? You should have seen it when it was, back when an attacker had 30% of their own damage returned to them as magic damage.
Current thornmail returns 10% of your bonus armor + 25 as magic damage on auto-attacks. This is beyond insignificant as if you have 300 armor, you deal 55 damage back to an attacker. I faced a Darius as Vayne top and he had thornmail and I simply murdered him anyway with 10% lifesteal - the wounds/reflect is so pathetic it really does nothing.
Even if you had 400 -bonus- armor the amount of damage taken per auto would be 65. If you have 200 AD with 32% lifesteal you'd pretty much heal for that amount per hit. The attack speed reduction is crap, the damage is crap, and the wounds cut 40% off the heal which is easily fixable - add more attack speed!
I've found Frozen Heart and Randuin's Omen to be far more effective - attack speed reduction permanently in an area around you and Omen for 20% damage reduction off crits with the active slow. Remember, if someone is dealing magic or true damage it won't be effective. Thornmail only applies damage off basic attacks dealing physical damage, so there are better options right now unless Riot reworks it.
It's not, it's a nice niche item against life stealing champs as it should be, thorn will only actually hurt you if the person is playing rammus.
It's not OP. The reason it isn't is because there are so many % health damage items and abilities as well as true damage skills and masteries. Champions like KaiSa, Varus, Vayne, Kog Maw, and others are not impeded by thornmail nearly at all.
Thornmail by itself is kinda worthless. you need other armor items to make it somewhat good. such as 
or 
I almost never build it because it only applies grevious wounds to whoever auto attacks, not to mention the INCREADIBLY short duration which was put on it to purposefully limit it to only reduce healing from autos.
And.... it doesn't give much hp, which is detrimental now that half decent hp is harder to get from runes and resistances being much easier instead. (Thanks to the new pick your stats system)
because of how niche it is. youre only really gonna build it when vs people who like to lifesteal or auto you a bunch, but even then something like randuins would be better if theyre strait auto attackers.
plus, randuins grants more health then thornmail, and has auto attack damage reduction unlike thornmail, and has a soft CC active to boot, with both being the exact same price
I agree, it's quite powerful. Magic resist, though, counters it pretty hard.
lol thornmail is only really built if facing more then one auto attacker.
and i rather get Randiums over thronmail cuz it offers way more in the way of damage reduction
