Thornmail
(Woah, long post! There's a summary at the bottom if you want)
I've been seeing several "Buff Thornmail" or "Thornmail has been gutted" threads lately. From what I've gathered, the general argument is that Thornmail is no longer the go-to answer for dealing with adc's.
What's the purpose of Thornmail? To make it a bad idea to focus you, but SPECIFICALLY adc's. There's already two other items that are very anti-adc: Frozen Heart and Randuin's Omen. These two items reduce attack speed by 15%, and two of them offer TEAMFIGHT UTILITY.
What does Thornmail offer? Return damage for better dueling against adc's.
Thornmail is a selfish item. If you're arguing for Thornmail to be reverted/buffed for SUPPORTS to use, then you aren't really supporting.
That takes care of the support audience, which leaves Top Laners and Junglers
I'll start off with a little anecdote. The day I saw the change to Thornmail in the patch notes and read over what exactly the change was, I couldn't help but think "Oh man, this is gonna make Thornmail so good for armor-stackers".
**The results were beyond what I expected. **
Here's the thing, old Thornmail COMPLETELY depended on the enemy auto attacker; You had no say in how much damage it put it out besides buying magic pen, but that wouldn't make you very tanky.
With the new Thornmail, it allows armor-stackers snowballing potential; If you get fed, you get MORE armor, which gives MORE damage.
It's not just ANY damage; This is damage towards ANY UNIT. What does this mean?
- Faster Jungle clears
- Minion Wave clear (Need them to focus you)
- Damage to ANY TYPE of champ that auto attacks you
Before you get all up in arms about this, there's some things to take note about the Thornmail patch.
When I saw Thornmail and Warmogs, I looked at it as Riot creating two new and distinct builds for people: Armor-stacking and Health-stacking, respectively.
Armor Stackers: 
Health Stackers
etc
Do you see a problem here? There's not many champions that get damage based on armor.
The new Thornmail benefits those who stack armor more than those who stack health. To those who just wanted to slap on a Thornmail onto their build and be done with adc's, you just can't do that anymore. But remember that Thornmail was NOT the only answer; You still have Frozen Heart and Randuin's!
Summary:
Thornmail has changed, and we need to see these changes for what they are.
It is no longer a standalone item.
You can't just grab this item to-go when there's a problematic ad auto-attacker on the enemy team. This item is now for those who stack armor items to complement the new bonus-armor ratio on it. Speaking of which...
Thornmail is now for armor-stackers.
The return damage has been halved, so most of the impactful return damage comes from stacking armor to make use of the bonus-armor ratio. Who better to stack armor with than the three most notorious armor-stackers around?
Remember that it's not the only anti auto-attack item out there
Thornmail may no longer be a stand-alone item against auto attackers that anybody can just buy and make use of, but there's still Frozen Heart and Randuin's. Besides, these two items cover on-hit champs like Ekko, Diana, and even the notorious Vayne!
If you've read everything, thank you for your time. I'll try to answer any questions that you guys post here, but please try to keep it respectful!