I think a good MMO is entirely dependent on the players' freedom within the game. Lately, themepark MMOs are designed specifically as linear, sometimes slightly branching RPGs. Back in the day of Ultima and Tibia the players essentially had a bunch of skills that they could train from start and that was the driving force behind playing. What you did around that was up to you, buy houses or explore the depths of Kazordoon or just sit out front of the depot chatting with the other folks making Ultimate Healing runes.
The developers added content such as new continents to explore, new creature and bosses and occasionally trigger unpredictable events like Orc raids in Thais but overall the developers themselves exerted little control over the players themselves. This is my opinion, the greatest failing of modern MMOs. They remove the unpredictability of human interaction by gorging the game with scripted NPC interaction and pushing the focus towards a prewritten story, rather than the players' stories all coming together to make a world.
This is why EVE Online continues to make news, it's not special when a raid kills Arthas but it's special when 2000 players accidentally initiate a battle that costs $25k real money. So I think it's a bit of both, the developers need to adapt the game to the freedom players require and the players need to adapt to freedom they're given. Unfortunately, most MMOs choose the themepark approach these days, forcing you down a linear path to level cap where you just sit for ages doing the same thing with little to no character progression.