When I first started Achaea, I made a sylvan who immediately had an effect on the world around him: he could heal destroyed areas of the forest, and he helped design a library! In fact, if you visit the Sylvans House estate on Achaea now, his rooms are still there! That is having an impact on a game: real, measurable, and immensely satisfying.
FFXI had satisfying moments: getting that final drop, for instance, or finally beating a difficult boss. But that sense of accomplishment quickly fled, and I was left looking for something else satisfying to do. In Achaea, every little event can be potentially satisfying: roleplaying with your friends for an hour can create or dispel drama that lasts for in-game decades to come.
Final Fantasy also had little to no player-versus-player interaction. There were three city-states, but the rivalry between them was at best friendly and at worst pointless. Achaea is completely the opposite: the factions are distinct and in general oppose each other vehemently, creating continuous and compelling conflict, with both the clashing of swords and the lashing of tongues. With over a dozen different Divine in Achaea, the larger factions can be split even further into small groups
My current character, a priest, had his city destroyed during a terrifying event in Achaea. For two months, we've been roleplaying refugees -- those who have no place to go. Our Gods (administrators and game designers) took our suggestions, and now we've started building our new city. We are watching roads being built, and we have to defend our labourers from our enemies who want to prevent us from rebuilding. Nothing like this is remotely possible in a static world; no "area events" like Guild Wars 2, Rift, or Final Fantasy XIV's new FATE system, which are designed to make areas seem less static, are even comparable.
If you love playing games with depth that you can actually make (and keep) your mark on, you're looking for Achaea: Dreams of Divine Lands.