I can't really pinpoint one specific reason. It's likely a plethora of little reasons that sum up and make the experience not worth playing anymore.
I quit once before (but came back after a year), so going off of that, I think I know what pushes my buttons the most:
- absolutely devastating RNG grind (original atmas style)
- extremely toxic raid scene that I am involved in
- inactive friends
- a feeling that gear grind is a waste of time
That said, at the time the game was a lot more bare-bones and my circle of friends a lot more limited too. Nowadays there are many other systems in place (glamour and housing, namely) and so much other stuff to do and so many other ways to obtain decent gear and all that, that a stupid grindy quest alone would not have the same devastating effect. Some friends quitting has no big impact anymore either because I know a lot more people
The one thing that remains a big danger to my enjoyment is RNG. I don't get bored making a plan for months down the line on tokens/currency. I don't feel "excited" when something RNG finally drops, I just feel full of anger and hate at the thought that it might as well had never dropped. Currently, RNG is...kiiind of bearable (3.4 was pretty borderline to me), but if it became worse, especially involving things I really want and care about (so fluff like hairstyles, glamour, furniture, etc.), I don't know how long I could bear it. My tolerance for RNG is really, really low >.>
On the topic of the game going F2P or more cash-shop centric:
While I absolutely do NOT want that, I have to honestly say that I doubt I'd quit over that. I come from a F2P background, I'm used to just about everything, including pay2win, and am patient enough to actually do stuff ingame instead of sinking money into buying the best gear, buff scrolls and whatever.
Not sure about new MMOs getting released: FFXIV is pretty much top of the line in terms of polish now, and whenever I check out another MMO I quickly return to FFXIV in a matter of days simply because I can't stand how messy they all feel, their interface, systems, etc. u.U
In terms of content, lore, visuals and all, it's also pretty much right up my alley, it's hard for me to imagine a game that fits that even more (except for a graphically impressive vanilla Mabinogi..but that's probably not gonna happen again..xD). I also just really don't like starting over, and any new MMO would require that of me, learning all the items and skills and balances and fanwikis and whatever anew. Too stressful and timeconsuming. I'll put that off to the point when FFXIV shuts down and I HAVE to migrate somewhere else..^^"
Oh, but just like briarthorn, losing my character would make me quit in a heartbeat.
I know it's fast to level now and all, but a very big drawing point to MMORPGs in general is how incredibly invested we get in one character, and that's more true for FFXIV than any other MMO because you can do everything on a single one, no alts required.