I must be missing something but how come so many people are saying that CoP was messed up? I found it to be one of my favorite moments from FFXI, it was a challenge and it felt good when my group was one of the first to get through. I play AION now and it is ok, but there is no real challenge, I like the fact in FFXI that you had to achieve Sky and Sea. I hope FFXIV will bring back the challenge.
Multiple facets of difficulty got CoP the thumbs down from a lot. The difficulty of the missions themselves (pre-nerf). The difficulty of finding the perfect group if doing it PUG style. The difficulty of actually even getting to attempt it after the rush died down. Level caps and needing level cap gear before Sync came around. Simply how long some things took like farming Mistmelts, Anima, or slogging through level capped labyrinths with TS mobs. The fact you'd pretty much have to start over again or wait 2 hours if you failed.
That's just touching on the mission progression. Nashira aside, Limbus and the ABC system was pretty good. Sea, to this day, still needs some work even with pop items from jailers being 100% now. Thinking mainly money drops or slightly more robust equipment options like a blue moon chance a piece of Homam or Nashira might drop. AV. People also disliked how a lot of the zones were locked off one way or another if you weren't so far in the mission line. In terms of other expansions, this only been the case with Sky, Dynamis, Einherjar, and possibly Walk of Echoes if SE remembers it's in the DATs. CoP also lacked new jobs, which when people are looking for fresh gameplay, it's one of the best cards you could play as a developer.
I can understand some might look back it fondly, but all those little things added up for others who weren't fortunate enough to have perfects schedules for statics or an army of friends willing to blow EXP and money on meds to help others play catch up.
Pretty much because the missions were like, the only thing to do for a long time after the release of the expansion. If you were unable to do them, you were out of luck. At least the recent expansions catered to almost everyone instead of leaving out a huge group of people.