Thornmail is a binary item, and that is why it is "weak"
Everyone there complains about how Thornmail has been nerfed heavily with the last change, and that it needs a buff because it is unviable on any champion not named Malphite, Rammus, Nasus, or Poppy, but I disagree. Thornmail can't really be buffed, especially now that ADCs are weak.
The problem with Thornmail is taht this item is very binary. Back when it was strong, it was a hard counter to ADCs, because just by purchasing this item meant that they couldn't target you at all. They would die because they were doing what they were supposed to do; auto and do damage. Essentially, they were punished for doing the right thing. To some extent this is still true nowadays, but you are required to stack heavy amounts of armor to make Thornmail worth.
Meanwhile, this item was, and still is, useless against mages. They laugh at this item's armor and they don't auto to deal damage. At least other armor items are somewhat useful against mages, for example, Randuins, which can slow a mage with its active (and its HP is useful against their burst). This item isn't that good against AD casters either, because those don't rely on their autos and they tend to stack heavy amounts of armor penetration.
I disagree that Thornmail should be buffed, buffing it will not solve its problems (this item will still be useless against mages), and will actually worsen the problem of crit ADCs being weak now. It will actually further encourage the use of lethality ADCs, since those don't rely on their autos to do damage, and they stack heavy amounts of armor penetration.
I also don't understand why people suggest things like "make Thornmail apply Grievous Wounds" this would break this item, and also would cripple non-ADC champions which accidentally click on a tank with this item, such as Swain or Vladimir.
What Thornmail needs is a rework. I don't really have any ideas but I hope that Riot has some nice ideas for Thornmail when the tank rework comes. The only real idea I have is to change Thornmail's passive from an active instead, it would be allowed to be heavily buffed because you would have windows of time where you don't have the passive and your opponents can play around it. Also, it would also allow Riot to tweak this item's stats.
- If you auto them you are going to have your attack speed reduced by a small percentage but you are not suiciding by attacking them, they are still a safe target.
Also, this item is useful against mages because its active slows them and its HP helps against their burst.
- You are only taking damage if you are into their range, not if you auto them. And as an ADC you shouldn't let a tank be in range to damage you with Sunfire Cape's aura.
Also, this item is useful against mages because it does damage against them and its HP helps against their burst.
- You only take damage if they auto you. It is physical damage and ADCs gain armor per level.
Also, this item is useful against mages because the damage and slow portion from its passive still works against them. And HP helps against their burst.
- You only take damage if they auto you AFTER they cast a spell. It is physical damage and ADCs gain armor per level.
Also, this item is useful against mages because it slows them and it has cooldown reduction that let you cast more spells against them. However, it admittedly does nothing to reduce their damage.
- Like Randuin's Omen, this item does not do damage at all and its main purpose is to slow enemies' attack speed. Unlike Randuin's Omen, however, this item does not have anything that helps against mages aside from the fact that by having more cooldown reduction and mana, you can cast more spells against them (and by more spells I mean, CC) - At least this item has some use against mages due to that reason alone, unlike Thornmail, which is utterly useless against mages.
and ignore 52% more of it with one mastery and 
is actually not so oppressive vs adc
magic resist & lifesteal this is arguably the best thorns counter item and is often bought for the active alone which helsp vs the tanks cc
similar to scimitar for countering thornmail with mr and a bit of sustain during passive and passive also counters thornmail and most of tanks damage
ad item with mr and a useful active helps vs tanks cc like scimitar
more mr
12% reduced damage this includes thorn mails return damage
and other items built from it as the lifesteal directly counters the thornmail damage
still around
Even just 
better on champs like 
, she isn't good enough wit it... "
already works on Syndra R
's bonus maxes at 1000 health difference, so…starting from 1001 health, I'd assume? It's still the same bonus, but, compared to your heath total, it becomes slowly weaker.
and 
(primarily, because most deleters are AP), assassins get whatever item they need to give moar damagez, and marksmen get the two 
and choice of boots on most current ADCs then Thornmail feels pointless