As I was writing that last Rift post, I jotted down several things that have led me to return to Rift over and over.
I used to have a huge "this is what I require in an MMO" list that was based on everything that I'd lost when SWG went under. Having played so many different games since then, my list is smaller, but I think for me these are the qualities I'm looking for:
Characters you like to play. This is a combination of how they look, how well their name fits them, how successfully you are able to fight, and what abilities the class has.
An engaging world. We're all explorers here, folks, and traveling through any world for the first time should be a pleasure and a grand adventure. Cities and landscapes should be distinctive. You should be seeing something you will never see anywhere else.
An interesting and useful crafting system. There should be a treasure hunt feel to finding resources, you should be able to make decorative things for your character and their homes, and armor, potions, scrolls, jewelry, enhancements of all kind that make the game more fun to play through something you've created.
A variety of ways to level up. Different paths through the world, extra xp from resource gathering, dungeons, public quest systems, crafting, the more ways you can make the route to the top fun, the better.
Music plays an important part in setting the mood for a world or making combat more exciting. It should never be unnoticeable or set you on edge. It should welcome you in whenever the game loads and become part of your memory of adventuring in imaginary lands.