The scale and scope of a fun MMO is almost unmatched. Think of a good RPG. All the complexity it had, the character customization, the stories. Now multiply that dozens of times.
A good MMO will always have something to do: a short quest for players to complete, a resource to be mined, systems to be learned and gamed. Logging in after placing an item for sale on the in-game auction house, to see if it sold is a unique MMO experience.
A breadth of challenges means players can play the game at any pace. If they’re a solo player who likes beating up rats, they can get whacking. But players can grow and join large raids or do everything in between. They might even join the game without a single person on their friends list and end up with lifelong acquaintances.
They’re also just amazing places to explore. The open world gives players a reason to go somewhere, and many games will reward players for discovering new areas or finding particularly impressive viewpoints.