What led me to eventually leave the MMOG's I've played is one simple thing: not enough diverse stuff to do.
As a casual player, I really only have time to invest in a single character in one game. That means that I really only can fight one kind of battle using a very narrow set of battle skills/talents, and I can only pursue a certain range of crafting avenues. Since the socializing and lore-reading of a casual player is very limited, there just isn't much diversity available in terms of actually impacting my character and enjoyment of the game long-term. All I really have to look forward to is doing the same small set of things over and over for the long-term. I can only imagine how boring such games must be to hardcore players.
Which brings me to the question: why isn't there more breadth in these games? Why are there so few alternative areas of advancement? It seems to me that MMOGs could offer nearly infinite lines of potential character advancement, not only in battle and crafting, but in all sorts of areas, like politics, an actual lore-based advancement system, professions where one can open up a shop and teach other players that don't want to invest advancement time in the profession (such as, new animations, new pet designs, new gear designs, house and furnishing designs, etc.), exploration advancement that opens up areas of the game, language advancements that opens up all sorts of new avenues, personal storyline advancements, city quest advancments that moves you up in rank and/or fame in particular cities or even particluar areas of the city - or even, just personal relationships with NPCs, where you don't know how "whom you know" and have helped out might help you in turn in the future.
One of the things I really liked about EVE was that there seemed to be an endless assortment of areas one could study in, that each branched off into many other sub-specialties. It left me wondering about the sheer lack of breadth of advancement systems in MMOGs (and, truthfully, even in EVE, since basically all advancement was in a fundamentally narrow meta-system).
IMO, adding new lands, bosses or gear is not having "new" stuff to do; it's the same stuff you've been doing, only wearing a different set of clothes. This may be adding depth to the current systems, but it is not adding new and different systems. The potential is limitless, and IMO largely untapped.