Thornmail is no longer countering AA's since its nerf. You need 600 armor to achieve the same result as previous version which is extremely rare. This item has lost its effectiveness.
If you want this to counter AA only then you will need to increase the scaling from AA. If you want this item to be more utility based rather than passive then add an active that doubles the scalings with decent cooldown.
One vayne died to one Thornmail before worlds, gg Thornmail.
Playing a tank is a joke unless its a juggernaut. I remember when every game had a tank and adc's did fine against them hell most of the time teams had 2-3 tanks and this was before vayne. It was slightly slower yes but at the same rate the tank was a threat to the back line who usually didn't build resistances. Now building resistances is almost a laughable joke. Thornmail was a hidden king for so long its sad to see it die just because riot doesn't like how tanks slow the game down. Also why keep the two damage kill scaling items but remove the health one because its passive was op? The health at the time wasn't even better than warmogs and when did you see a tank go 10/0 in a game back in season 2? Botrk has been the best lifesteal item since season 4 simply because it just deals the most and heals the most damage when it comes down to what adc's do the most autoing. The lifesteal after a recent change where the hp% heal back was buffed made it so that the item countered thornmail before they changed the item. Nerfing Armour because of 4 champions you decided to release overloaded like usual is just down right insane. Also whats up with all the hp% true damage or armour ignoring champs now. GP ignores 50% of your armour, Cait can ignore a large part of armour on certain auto attacks. Vayne has always done %hp true damage. Seriously why does this game have to have so much true damage? Yet you go off and nerf things like armour to make sure that no one would ever want to pick it up because tanks slow the game down and fights might last more than 5 seconds. Guess we should of saw it coming when turrets no longer did damage past level 6 though so our bad right?
Whoever thought it was a good idea to change thornmail, well, it wasn't. I only built thornmail as an armor alternative to make a high AA comp regret it, and then maybe buy a bunch of health or maybe some other armor, but forcing it to need more armor to work well...uggh.
Yeah armor scaling is silly because if the armor scaling is at all high, supports/mages/other tanks will take more damage than they deal trying to auto you, while it still won't be very effective against adcs. Sure it is weird that thornmail counters adcs who are doing well, but they can always build magic resist/bloodthirster. Now thornmail doesn't really counter anyone, because the reflect damage is pretty low, it leaves you highly vunerable to magic damage/true damage since you are paying 2300 gold for only armor, and most champions can avoid autoattacking you unless you have a taunt.
It isn't suppose to be a counter to auto attacks.
And I can feel the downvotes now, but it's meant to add more DPS to tanks, not to make you immune to an entire roles primary damage output, that's just crazy.
600 armour?
Easy.
and let's not forget the all the lifesteal items that I never see any AD champs that plan on doing any amount of damage go without. I hardly ever see an ADC not grab 
as soon as they can. Now with the 
nerfs, getting BT is just overkill. Honestly I just get 
and 
now. gives more armor, more health, a speed boost AND I do magic damage in an AOE that way.
It suppose to be better againts sated devourer. People just dont realize that now
Simplest solution to making Thornmail good again: BLADEMAIL. Make the damage reflect component an active instead of a passive, with a noticeable effect so that those ADC's know they cant attack that target without killing themselves.
Blademail works in DoTA because while it is EXTREMELY powerful, it is also very obvious as to when it gets used. So it can pass with a 100% damage reflect as True damage.
But Riot probably just wants to nerf tanks and anything that slows down ADC's and assassins at this point. Honestly just feels like it from their recent balancing.
Idk what they were thinking with the nerf. The item is useless right now.
you'll probably get more countering from tabi and randuins with the -10% physical and crit damage. As a avid tank top/jungle/support player RIP
I have never bought after its nerf. Not worth anymore. 
I like the idea of giving thornmail an active, though I'd like to see something more unique than just doubling the passive. Maybe make the active increase the passive an amount proportional to how close you are to the champion attacking you. Like if you were in melee range the passive would be doubled but at 800 range the active wouldn't provide any extra effect. This would give some strategy and also give the ADCs some counterplay (rewarding good kiting.) This active would also make thornmail even better at countering melee devourer junglers.
But "the same result as previous version" was horribly broken and should never have been a thing, so...good. That was the point. They didn't WANT it working the same way as the previous version. The ADC should not be killing themselves from Thornmail damage alone just by attacking a tank.
Dunno what world you are living in... i use it and it is even more effective now than it was before, at least it is in the early game. It does fall off a bit in the late game but I believe that was the entire reason they tweaked it. It was being picked up to often by anyone in an effort to thwart the enemy adc or other champ that was countered by the item. Now it is an item that works best only on the champions it is intended for, tanks.
