@Riot: the thornmail changes were a terrible idea
the armor scaling in particular, and here is why.
the basic thought behind changing the passive is clear. splitting up the passive into a % damage return and a flat damage component was supposed to make the item work better against heavy attack speed/on hit builds. like devourer junglers and attack speed carries, and even magic damage dealing carries 
etc.
this in itself was not a bad idea. the item would be as effective against crit heavy builds as against on-hit builds and not punish a particular type of carry heavier than the other.
having the flat damage return scale with armor was supposed to make it more effective on champions who itemized specifically into armor and to survive physical damage.
but here is the problem with the second part. the armor scaling on the item creates extreme edge cases of anti auto attack/armor heavy tanks like 
and technically 
these champions are geared towards achieving much higher armor values than most other tanks and fighters in the game and have additional tools to counter auto attacks. a full build malphite can achieve more than 600 armor, while most other tanks and fighters will get around 250 to 300 armor even when itemizing heavily into it.
in fact in my last malphite games just before the end of season 5, the old masteries allowed me to almost accomplish 800 armor by lategame in some of my games, with health values around 4000 health. (2 or 3 randuins stacking for raw stats and crit damage reduction).
what does this mean for thornmail? well, if the item was good on any regular tank or fighter, it would be absolutely **insane ** on one of those three. the nature of thornmails passive itself already makes it better with increased armor in your inventory. the additional armor scaling on the flat damage return portion of the passive only amplifies this problem. so the thornmail in its current shape has to be balanced around these heavy armor scaling tanks, which makes the item feel almost useless on anyone but those three.
i think thornmail needs a really good look over, and its scaling changed to be less armor dependent and overall much more effective on other champions.
especially with the addition of extra lifesteal on items like 
and the introduction of 
the amount of lifesteal carries can stack up is almost comical right now. it doesnt help that two of those items are adding a noticable amount of mr which further negates thornmails damage return.
maybe instead of making the passive scale with bonus armor, it might be a better idea to have it scale with bonus health which is more of a universal defensive stat. and even the difference between heavy health stacking tanks and fighters like 
or 
to regular health stackers isnt as large as the difference in armor between a regular tank and someone like
tl;dr: thornmail is basically trash on anyone but malphite and rammus after its changes. because if it was any better on other champs it would be broken on those two.
it needs to get fixed.
+ your 
= FE FI FO FUM! I SMELL THE BLOOD OF A LOWLY CARRY! And then he'll be nerfed to useless. again.
with a working Thornmail and 
running straight at you. He'd be unkillable, unkitable, and dealing tons of damage without even touching you. No Thornmail is sadly unhealthy this day and age and not because of tanks, but because of other classes abusing it.
Extremely general item with powerful stats. Lots of life steal and gives a shield that can achieve multiple purposes.
) which allow them to reduce Thornmail's damage and thus they require much less lifesteal than what the calculations suggest. In addition, some ADCs are capable of bypassing Thornmail's damage with their abilities or on-hit effects - for example, Vayne, Lucian, Graves, etc.
