There were 4 main things that made FFXI fun in my mind and many of the game’s systems and events were built around them: preparation, coordination, tension, and flow. Events like HNMs, Dynamis, Limbus, and Salvage certainly had these things, but Abyssea was the height of FFXI in terms of game systems in my mind: it brought together and refined the best aspects of all previous events. People disliked Abyssea because of how the gear (coupled with the level cap raise) outshone rewards from older events, but the gameplay itself was great. Voidwatch, Legion, Delve, Escha and other events afterwards were interesting as well, but never quite reached Abyssea levels in my mind.
Preparation
Getting your skills capped, leveling subjobs, putting gear sets together, arranging an appropriate party, finding a camp and getting everyone there, etc. was great fun and required a lot of thought and time to get right. Proper preparation could mean the difference between a crappy leveling experience and an incredible one.
Abyssea: Abyssea added special KIs (essentially a parallel progression system), special camps with monsters that had various strengths and weaknesses (this was always there, but felt more acute in Abyssea), pop items, benefits to bringing a varied party setup, among other things.
Coordination
Once you had a party set up each person had to play their role effectively in sync. The DDs had to manage their enmity with the tanks and skillchain with eachother; the puller had to find the right pace of bringing new mobs to camp to keep the chain going, and so on.
Abyssea: New things like lights meant that killing blows had to be planned (magic, physical, or weaponskill). For NMs people had to plan pop item acquisition and communicate to find weakness triggers.
Tension
In FFXI there was always some risk involved in all activities. If you didn’t prepare and coordinate properly you risked wasting a lot of time building a party that ended up falling apart before anything was even really accomplished. In the worst case scenario you’d end up losing experience points or even levels.
Abyssea: The big looming threat in Abyssea was that of the time limit running out. If the party didn’t quickly find its bearings then people would risk being kicked out and losing their progress for that run.
Flow
When the preparation and coordination came together to overcome the tension just enough then FFXI’s party gameplay became like a well-oiled machine. A bad party could be extremely frustrating, but a good one made time fly by. Getting into a flow state was incredibly rewarding and was quite addictive and in my opinion it was what made FFXI great.
Abyssea: "Oh hey look, at first I was worried we'd be kicked out, but now I've got over 1000 minutes worth of time remaining!"