I have to agree with the majority here, the challenge in XI was the time spent waiting. My first MMO was XI, from UK release until about a year before Adoulin released, and I still have very fond memories of meeting new friends and going on adventures with them, enjoying the story. However I also remember the frustrations, waiting in jeuno for a exp party to try and get a level before the weekend was over, failing to setup my own parties because members wouldn't go without a BLM (pre-ToAU) or a RDM/BRD (port-ToAU).
Even trying to do missions, I had fun when they actually underway but the preparation and travel dwarfed the time spent fighting. Some content took some thought but a lot just boiled down to a zerg or manaburn.
Endgame I found to be the biggest chore, SO much waiting around and you could spend weeks without getting anything out of it, and the fights were not all that satisfying.

This waiting pattern was the same through each expansion, and yet I still loved that game but given the choice I'd stay with XIV. The pattern of collecting tomes maybe th same but the content changes, I get to enjoy it in a long or as short as a play session as I choose. Overcoming content gives me great satisfaction, and i have a tangable means to see my progress.

Sorry about formatting, on my mobile.