The threat minigame was something that was completely broken at the high end. Threat capabilities of tanks were fully realized somewhere in the Tier 5 level of content/gear, and there was 2 entire tiers higher than that. All of T6, and then all of Sunwell. When DPS starts getting the extremely huge upgrades of those tiers, they just shoot past the tanks in TPS output.
Something absolutely needed to be done, the game was no longer fun at all for high end tanks because if your DPS is aggressive and pushing it to the limit as well, they would always simply blow by the tanks in threat. It's emphasized on short-lived trash mobs where DPS feels more confident in attacking things quick because it's just "trash" and getting yourselves killed on it won't wipe the raid like a boss. Example like, sheeps are up, tank moves to break one, a second or two later it's being nuked hard, at that point you're pretty much going to have to taunt towards the end to keep it stuck to you while you get the next mob broken.
I've always had extremely exceptional threat generation, all feign-modesty aside, I had the highest threat reports for most of the good "threat gauging" fights for the longest time, pre 3.0. But there exists fights where threat is even more sensitive and fast-DPS is essential, where you simply can't wear that same type of threat gear. Specifically, a fight like Felmyst or the epitome of it; Entropius [M'uru phase 2]. Prior to the nerfs Entropius required a hardcore raid DPS burn of every consumable, timer, heroism, etc and you had to kill him in about 90 seconds, max. Meanwhile he hit like a truck, and the parts of the fight prior to that also put immense pressure on the tank's suvivability, so you couldn't just walk in there in a full threat suit and tank Entropius phase 2.
As I tanked through the different tiers of content, I saw how my threat gains in earlier stuff started slipping away and becoming dangerously frustrating in the higher tiers. At the very highest tier of fully Sunwell geared DPS vs the tank, it was no longer fun. No matter how good you are, it was hopeless; dps would always annihilate you on threat, and thus, they had to gimp themselves to even keep things manageable.
I could truly write about how broken the threat scaling mechanics were prior to 3.0, for pages and pages, but it's fixed now. I'm very thankful for the changes and glad they realized it.
On the flip side, while threat is much easier to manage now and no longer something that fully encompasses the game for the tank, they made it equally engaging and fun by giving us more abilities, more on-proc decisions to make, more "survivability" reactionary cooldowns, and great DPS output with very high damage output, high damage is always fun to see.