Your first MMORPG is almost always 10 times more addicting than any subsequent MMORPG. The 10 million or so people that were sufficiently into videogames to play WoW, but no so into them as to have played MMORPGs before have now had their first MMORPG experience.
I dont agree with this at all. Quite the opposite; the new MMO experience is addicting regardless of how many you've played before.
I started with Daoc, and damn that was addicting. Played it heavily for ~2 years.
Then I played SWG for its first year, and even that was fun for a while. I hologrinded with the rest of em. Then WoW. After (years of) WoW I played EVE for a year. And that can also be addicting in a different way. EVE draws you in. And now I'm back casually playing WoW and planning to play Cataclysm.
So the addiction factor has nothing to do with what came before, or comes later, it has to do with what you are playing *now*. In a few years those 10 million people that started with WoW, the gateway drug, will instead be hooked on the shiny new MMO. All that remains is for someone to create a WoW killer.