Ah, I remember the GTS. It was actually pretty good, it spanned all servers, and nothing was really overpriced, and prices stayed pretty stable, minus when an item was chosen to be merched by the community. If I wanted prayer pots for cheap, I could just set a lower buying price before I go to sleep, and wake up in the morning to find that I got some

You wouldn't have to keep checking the MB for cheap prices like you have to with the MB system atm.
Atm, some mats can fluctuate quite a bit, and you can't check the price of something on all servers easily.
The GTS imo was actually better compared to the FFXIV market board, I remember how much I missed the GTS when I first started playing back in ARR.
Also, those who say FFXIV is player controlled? The GTS was player controlled also.
I think people disagreeing here have never experienced the GTS first-hand. It was so bloody good, knowing that what you craft had a certain, stable price, whereas the sell offers people did would give more power to the buyer, preventing stuff from being too overpriced, like some items are now in FFXIV.
The GTS system was honestly a better system. You could also put up a stack of 99 on the GTS, and instead of someone else being forced to buy that entire 99 stack, people could buy what ever amount of your 99 stack they wanted! It was so useful! Since it wasn't a hindrance for someone to sell large stacks, people didn't have to undercut so much to sell them, which meant more players could sit down, make a lot of something, have them reliably sell, which actually made prices more reasonable since this nice system promoted more crafters/suppliers to sell things.
Also the anomynity of items being sold! Market trolls couldn't find out what items you were selling and ruin your business.