I've tried dozens of MMOs, and still nothing comes close to capture the experience, spirit, and atmosphere of Asheron's Call 2. Open, seamless world, dropping items onto the environment, unique races and classes, etc.
But the crucial difference for me why those other games don't hook me: lack of suspense in combat. So many games' combat is based on trite, mindless, repetitive action. But action by itself doesn't connect deeply with the player.
What is missing from these games is suspense, which AC 2 somehow was able to evoke. Kind of hard to explain--it has to be experienced personally: a real sense of dread, of impending failure, of "oh so close, please don't die!". It's an experience that cannot be emerged by endless action and quick reflexes. There has to be more than just action; memorable experience (in games, books, movies alike) has to bring about a sequence of strong emotions, like peril, fear, belief, celebration.
Alas, it's apparently almost impossible to find the same feelings in other MMOs (the many that I've tried) as I did in AC2. An essence of human experience that is absent, forgotten; something that cannot be implemented, replicated or replaced with better technology, skill, and flash.