Funny, I had a 1v3 as Malphite top with thornmail.
(If you know how to use replay.gg, look for the 1v3 at top lane near the end of the game.)
http://www.replay.gg/?r=NA1&id=1939393558&key=0qHcNwTg6mcFss5%2FSwovTFPIGTkwQmM4
If I had to say anything, the addition of the new bonus armor scaling for Thornmail's return damage allows armor-stacking tanks snowballing potential. Old Thornmail's return damage was only as strong as your opponent's auto attack damage, which made it the bane of most ranged ad's, some ad melees, but lent nothing against mages, tanks, etc.
With this NEW thornmail, armor-stacking tanks can get more tank AND damage at the same time without having to depend on how fed the enemy adc is? Hey, it even lets you proxy as hard as Singed by standing still as a rock, because minions will now actually take damage from Thornmail.
Point is, I don't see what's up with all the QQ threads about the new Thornmail when its "target" audience has been opened up to more than just one category (AD auto-attackers) of the damage spectrum.