The main issue with the new Thornmail, as I see it, is that it seems to have an identity crisis in its current form. Cold Steel screams "don't hit me, or I'll lower your DPS," whereas the Thorns passive screams "hit me so I can lower your healing." They seem to be conflicting, and those two together, on the same item, only really make sense on champions with taunts, and of the 3 that do, Rammus is actively harmed by Cold Steel's presence.
The tradeoff of healing reduction for less damage seems very fair, and seems like a much healthier niche for the item, however I would definitely do something about Cold Steel. If the issue is stacking Randuin's and Thornmail, is there not a more elegant solution?
Full Disclosure - I do play a lot of Rammus, so this is probably where most of this is coming from, but I think the criticism is valid still I believe.