I've heard the term MMORPG applied to small MUDs and EverCrack, so I suspect "massively mulitplayer online roleplaying game" is now a generic term that has unfortunately come to stay.
And in a sense, it's just as accurate for 100 people as 10,000. You can feel crowded in a small world for very few netizens, while a world that sustains 15,000 simultaneously can feel relatively empty if you've got enough activities and areas to keep them relatively separate. The important thing is to build a sense of community, a feeling that the game comprises a separate reality, and a secure environment.
It's true that MMORPGs lack plot, in general. Remember, the go-anywhere kind of CRPG tends to be like this, while the game with a clear storyline tends to be linear; the best traditional standalone CRPGs, like BG2 and Planescape: Torment effect a compromise between the two, and camouflage the linearity by throwing multiple sidequests at you which appear of a similar order of importance as the main path through the game.
MMORPGs can't really do this effectively. When I was working as a developer for one MMORPG for several years, we attempted to build large quests into our game. The idea was that we would run the same quest once or twice a day, in a special area, with limited attendance. The people who came in had to split up into parties, and generally start in different areas. We monitored all the parties, their activities and comments. If we ever got the sense that anybody had found out the details in advance from someone who had played before, and was trying to max out their experience and item gain, they were yanked from the area, and quizzed. If this was confirmed, they were banned from taking part, and anything they'd gained thus far was removed from 'em.
These quests were *extremely* DM-intensive. They had to be, because there was no way you could predict exactly how the customers would react in a complex, event-heavy area. It would have been much easier, IMO, to simply build more plotless regions, develop an elaborate new merchant faire, or even add a new gaming subsystem than run these quests--but the bosses wanted 'em.
And they were popular. Players with characters above a certain level really enjoyed the change of pace, the complex puzzles, the unexpected monsters, surprises, and the goodies they took away with pride. So...yes, themed plots are possible in wide-open MMORPGs, but they're damn hard to do.
