I actually thought the Defense cap was really cool. It felt like a serious investment in your tank's durability, differentiated you from other roles, and was a neat special tank stat.
I get the people saying "it's just stacking bleh", but idk, somehow it felt different to me than the dull hit-cap grind. I guess with hit-cap it's very binary: Whiff Mysteriously/Don't Whiff. Whereas with Defense every little bit felt like building a mountain one pebble at a time. Same with gimmicky high-Blocking builds for dungeon spamming.
It's similar to the talent trees. At the end of the day, yeah, there's no practical difference between clicking 61 times in a familiar pattern, or just having all those talents handed to you automatically behind-the-scenes. But there actually is, because just having something handed to you is different than "building" it yourself.
It's like your parents surprising you with a pre-built LEGO set and people going "Why are you complaining? You just would have followed the instruction manual and built it anyway", or suggesting we should get loot in our mailbox because "You just would've wiped a few times and killed the boss anyway, fighting was tedious and repetitive and predictable."
Gray lines and everyone sees it differently, but I think WoW has actually lost a lot of engagement value and depth in its attempts to be "logical" and cut away "pointless" things. Right ideas, but missing the more subtle aspects.