1. Social Interaction. Chatting. The IRC aspect gives it some social interaction, at least on the scale of say blogging, internet forums, etc... The emotes can be annoying, but add a big chunk of easy ways to have a complex interaction with someone in an RP setting adds to the role playing.
2. Role-playing. Definable characters that are very distinct from each other. The cosmetic aspects should help reflect that visually / audibly, but there should be more in the way the game is played that helps define characters from each other. It may not be truly deep role-playing, but if there's a good degree of thought put into it, then its satisfying.
3. Story / background. There needs to be a believable 'world' to adventure in, with a history that has lots of details. It may amount to lots of bland Foozle* characters, but as long as its consistent, it provides some framework. *Foozle from the idea that most quests in CRPGs are of the Kill Foozle variety (where Foozle is some named evil doer). The questing and other player activities should intermix with the background. Character development should intermix with the background as well.
4. Near fractal complexity in the world layout and design. Things should be very distinct. If the world feels like a plain desert, regardless if its forest, seas, mountains, etc... then it's a turn off. The world should surprise with lots of complex details.
5. Interesting player interactions. PvP for one, but also complex, detailed economy is another way to interact. A tale in the desert is a really good example of a strong economy in a MMOG. Guilding and social hierarchy and order is also interesting.
6. An interesting RPG mechanic. I think this is one that is not common enough in MMOGs. Many games tend to spoon off the most populist, simplistic systems, and fail to come up with a nicely thought out, customizable, and balanced system.
7. Relatively low technical overhead. If it can run nicely on a cheap PC system as well as a nice one, and logging in and out is smooth, then it will be more popular over all.
8. Relatively Cheap. Free or under $20 to start playing, and about half that to continue monthly, with no limits on # of hours played. The way I see it, 4-5 months of play should equal buying a new game. Pretty much if the MMOG is high quality, its the only game the user will play or want to play, then that's equal to buying 3 games a year or so.
WoW does most of these pretty well, though definitely not perfectly.