As far as garrisons go:
1: Lack of customization.
2: Convoluted and limiting adventure tables.
3: Repetitive and boring associated quests/invasions.
4: Damage to the existing game economy.
These 4 pretty much sum up my feelings. It felt like yet another over promise and under deliver from Blizzard, which is par for the course even so many years later. Blizzard is just notoriously bad at making fun mini-games, and even worse at making them interesting community events. I'm still shocked that after so many years we don't have an in-game player tournaments of any type, and there is an awful lot of potential for it as well as lore backing it up. They leave a lot of the community engagement to third party sites instead of having it in game, and I view that as a glaring mistake that's been long ongoing.
So again, what's the point of mining all that iron? What is it used for? What do you get out of it?
My point being that people don't go farm dungeons when they have everything from that dungeon. What I see from your idea is a whole lot of stick and no real carrot. If you make the carrot too juicy then people who don't want to do it will join in just for the rewards, and if it's not enough then almost no one will do it and it won't help with reviving the world at all. The only way I can see it being viable at all is if the materials have a separate market and are used for something more impactful such as player housing, which might be useful so it doesn't cause mass inflation of regular trade goods.
Finally, there is the worry about the systems that WoW is built on. There are tons of bugs and funky stuff in the game already, and a lot of random stuff ends up breaking even 1 expansion later for seemingly no reason. I just did a "horse and cart" quest in pandaria the other day where the horse took off randomly and the cart I was riding went sliding in random directions through the mountain. It reminded me of the
Skyrim opening glitching out.
I don't want you to take this as a "barrage of hate" as other people are saying. I just want workable solutions that will attract enough people for long enough to keep it relevant and desirable.