Having been through the World of Warcraft addiction experience myself (thankfully not as bad as "Michael" in the article), I would like to differentiate between two concepts here. On one side we have the general concept of videogame addiction, but I believe we should separate the more specific MMOG addiction from it.
There are two issues that make MMOG addiction different: I'd give them the working titles of "availability" and "character identification".
When playing any single-player game, you have the means to control your play-time: most (if not all) single-player games allow you to record your progress via saves or by dividing the game into chunks. The player can then stop playing for as long as they want (to go out with friends, watch a movie, take a vacation, whatever), then resume from where they left off with no adverse effects.
Unlike single-player games, you cannot exercise the same control when playing a MMOG. MMOGs start out simple enough - log in, check the sights, play a bit, log out - but soon you realize the downside to this huge persistent world you joined: it continues existing and changing even when you're offline. You then start to change your approach, increasingly relying on those Fifteen More Minutes [http://www.escapistmagazine.com/articles/view/issues/issue_194/5879-Fifteen-More-Minutes] mentioned in another article, because that guy who is helping you kill that monster for that quest will probably not be around if you log off now and come back later. By the time you get to the so-called "endgame" content, your gaming habits are altogether different. Instead of planning your play-time around your other activities, your whole day starts to revolve around playing. Quests are now raids, "that guy" is now a whole guild of guys and that means painstaking planning of play-time, so that 25 people can be online at the same time to take down a big monster. It's not a simple matter of "saving" your progress anymore - if you miss playing even one day, you risk missing an opportunity for getting "loot" from that monster, or an event like a first kill, or some such.
However, as a phrase often seen on the World of Warcraft forums goes, this all is "working as intended" - you cannot make a MMOG without a persistent world, and you cannot have a persistent world without the aforementioned problems.
Now for the character issue. On most single-player games, you're controlling a predesigned protagonist, whom you may or may not identify with. Some single-player games (eg. The Elder Scrolls series) allow for extensive character customization, but you're still playing your perfect alter-ego alone. Here's where MMOGs come in - you get to create a highly-customizable character, then you get to show him/her to everyone else. The show-off in all of us takes charge here, which means that it's not just a question of progressing in the game anymore, it's a way of feeling that dubious satisfaction of seeing your female warrior (whom you identify with even though you're a male accountant or some such - no disrespect intended) look better than some other bloke's female warrior. When combined with some degree of sociopath behaviour, MMOGs can be a very attractive alternative to the "cruel" real world.
On a similar note, there's the social rejection issue: your real-life friends don't like you cause you're fat/ugly/depressed, but your guildies love you because you're the best tank/healer/DPS in the guild. When the time comes to choose between an office party and a raid, the choice seems obvious. Granted, it takes effort to be the best tank/healer/DPS, but it's all done by playing a game, not by going on a diet or some other such chore, which is why in the end, the raid wins. Although I'd like to believe this is a MMOG issue, the recent article Crazy In Love [http://www.escapistmagazine.com/articles/view/issues/issue_188/5741-Crazy-in-Love] on The Escapist, and especially the disturbing material on "the FFVII house" and the "Sarah" case I eventually read by following the links have convinced me that single-player games can also exacerbate sociopath behaviour to extremes.
Last but not least, I'd like to point out that potential victims of MMOG addiction are not limited to people who are at odds with real life and looking for something better. As the article mentioned, for Michael the addiction started at a time when he was on the top of the world, and it was the same with me. What's more, before I started playing WoW, I actively opposed online games of all types because I believed their emergence was detrimental to the quality of single-player games. The reason I started playing WoW was because there was so much publicity - everyone was talking about it! With the benefit of hindsight, it's quite obvious that the publicity was intended to sell subscriptions, which it did, in buckets. I now realize that the MMOG publicity cogs are continuously turning, which is why even minor news concerning MMOGs are given extremely wide coverage.
In Europe, where I live, it is widely believed that the restrictions on cigarette advertising has helped combat smoking addiction among young people. If MMOGs were similarly spared the "glamour" treatment, maybe we'd have less MMOG addicts. As the article mentions, the medical community refuses to acknowledge "videogame addiction" as a valid pathological state. Think about how much revenue Blizzard, Sony et al. stand to lose should things change, then you might see why history always seems to be repeating itself.
