To me it's got to be a game where I can't see the end of it before I've gotten there.
WoW is pretty much the poster child for this. The gear, talent and questing system are designed to be so user-friendly, and so clean from a mechanics standpoint, there's no mystery to me. "Easy to learn, hard to master" is a phrase I'm kind of sick of hearing, because it delivers on the first part, but the second ultimately ends up being trivial. Who cares what mastery really means when 99% of it seems to be less about your understanding and more about your twitch abilities.
Compared to Everquest, which lacked a really clean, repeatable, reusable system, there were ALWAYS surprises. Unique kinds of gear. Really unique kinds of environments (that contained real hazards with real consequences.) Experimentation was a key part of that game.
On the one hand, WoW is almost flawless from a design perspective. It's so neat and tidy and modular and packaged, ready to be shipped, consumed and defecated. It's managed to be fun without consuming your freaking life like older MMOs.
On the other hand, it's hard to feel an attachment to newer games, because they don't really challenge you intellectually or really challenge your gaming abilities. Because companies aren't interested in making niche games these days, they want the holy grail. I don't feel involved in newer games like I used to feel with old ones. I feel like one more schloob treading a plotted path, more like watching a movie.
I'll stick with the next MMO that remembers what made the originals so appealing....where game play was constantly different and not trapped by the overall game design, where gameplay is allowed to emerge from the mechanics instead of being strictly dictated by them. I want to feel like I'm playing in a world again, not in some carefully monitored and regulated instance where novel things are rare and avoided because of the problems they bring with them.