I think the level of addictiveness will vary from person to person and their likes/dislikes about a virtual environment and what it offers them. <Sidetrack> For me, my MMORPG fascination begane with EverCrack...err... EverQuest. I actually joined on launch day, when it was dubbed NeverQuest due to the larger than expected playerbase slamming the servers and network capacity issues that plagued it for a few weeks. </Sidetrack>I stuck with it for about 4 1/2 years until Blizzard launched the behemoth that is WoW. I was in the beta for quite afew months and at first didn't see myself giving up EQ, as I was just to used to the gameplay. But it quickly grew on me. I was a die-hard player for the first two and a half years but slowly lost interest as they continued to water down the encounters and making it easier for everbody who could swing a sword or cast a spell to beat everything just to appease the growing mass of players. I still miss the days where there were only a select handful of guilds that would, or even could, take on Naxxramus and the other end boss battles of the day. I didn't much care for the Burning Crusades expansion, but have to admit Lich King was pretty good. I am curious to re-activate just to see how all the original content has been redone as a result of the Cataclysm expansion.
For me the biggest draw and what kept me coming back was the community, or more explicitly, guilds. It was a blast getting to know players from across the country and even world, planning/strategizing encounters, etc. That and some of the games tend to be like playing Solitaire. You keep playing, all the time thinking "ooo, if I just play for another hour I can get this done, and this, and...".