Now, realizing that this a loaded question and will be met with a lot of subjective thoughts, but that's entirely why I want it as well. Everyone (likely) has their own opinion what makes an MMO great. And can relate it to one they've played, or are playing, because of certain elements they find it them.
This is not a comparison against XIV thread, we've had many of them. XIV vs XI. XIV vs WoW, etc.
This is more about why do/did you enjoy THOSE games so much, that they to you, are more fulfilling than what you may be experiencing now?
For myself personally..
I enjoyed XI for how incredibly large it felt, particularly coming out of Bastok, or Windurst. It felt like Gustaburg just stretched on forever at some points. Leveling was a grind, but the party element was much more relevant and in some cases did build up comardrie between players, generally I imagine it's how all Linkshells formed, between people who partied, enjoyed each other, deciding to take it further. It's how I eventually joined my shell that I remained with until I eventually left the game.
I did not exactly enjoy the grind, but I expected it. Traveling was also much more interesting, while some may have found it easier to travel from a starting city to Jeuno that first time, I found it to be pretty rough.
Plus the fact that you had to actually buy your maps, rather than having them automatically given to you. While I enjoy that feature in XIV, the sense of exploration, it can go either way.
One particular memory was with a party of people I was with, at a lowly level 18, we hopped the ferry from Selbina to Mhaura, to explore, trying to get to Windurst. We didnt have the map for that first area right there (cant think of the name), so we're running blind, trying to avoid everything that could kill us, easily. We eventually got separated, I died, ended up home pointing back to Selbina. But it was still fun. Exciting, dangerous. I personally liked that sense of danger, that thrill.
GW2 was like that, exploring, but knowing that if I went too far (which happened), I could find myself overwhelmed and dead pretty quickly. It was fun to see how far I could go before eventually I got to a point I couldn't go any further till I was more equipped. Stealth mode was on all the time (not a literal thing, just saying, trying to be sneaky).
Never really played any others, did WoW for about a week, it was amusing, but I was still invested in XI at the time and had to choose one so I chose the one I was already heavily involved in.
Sadly, my days in XI got to become pretty boring, often logging into the game out of a sense of obligation, but simply standing around in my mog house for hours, doing absolutely nothing.