Honestly? I think Thornmail is fine.
So I've seen a bunch of threads ranting about 
and how useless it is and I don't really get it. Thornmail is a very specialized item meant to counter heavy auto-attack focused comps, like a 
jungle, 
mid and 
bot. In those situations you would be stacking large amounts of armour, including 
, 
and 
, which would both further reduce the enemy's damage but also increase the damage Thornail deals to them.
Some math to illustrate:
Say a Lucian with 100 AD attacks me. If I only have Thornmail, then it will reflect 40 damage (15 + 25) back to Lucian where original Thornmail would only reflect 30 damage. Combine it with Ninja Tabi and now I'm reflecting 47.5 damage (15 + 32.5) and have 12% DR against auto attacks. As the value scales with both enemy damage and armour, the value only increases as the game goes on and since it's calculated BEFORE armour pen, it won't be reduced by the Last Whisper line. Throw an 
on the late-game enemy adc on top of this and they'll kill themselves trying to auto-attack you.
Basically, while the changes to Thornmail will always make it most useful on champions that can naturally buff their own armour, they also made it much better at it's highly-specialized purpose.
Thornmail can be countered.
If the ADC has Lord Dominik + Mercurial, your Thonrmail is kind of useless.
Thornmail can counter only right click champions that have no way to ignore reisistances and with no way to deal decent damage lategame with their abilities- Trynda, Tristana.
And even they can deal with tanks with the right builds - bork, lord dominik, mercurial, IE, PD, boots.
Champions with true dmg on hit can afford to skip armor pen - Vayne, Yi, Fiora and still not die to the tanks, while killing them.
being present nearly every gaem further reduces thornmails effectiveness
