There's something about being online that manages to keep people captivated to the point where the world can go on without them knowing it. Whether it's the never-ending slew of information or the ability to communicate with anyone and everyone, when you combine the internet and video games, that's a recipe for a new addiction.
Having spent a good portion, if not my entire high school life playing MUDs (text-based MMOs) I made a deal with myself to not play any online games anymore. It's too easy to get involved, sucked in to the point where you lose all concept of time. I was never able to play moderately, mostly because these games don't allow moderate playing unless you want really slow progress.
Other than sleeping, there aren't that many activities that can keep me fixated for more than 8 hours, yet online games can do it. For my own health and well being, I stay clear of online games, not because I don't enjoy them, but out of fear of enjoying them too much. It's really too easy to let your life slip away and realize that days have gone by and you don't have anything to show for it except maybe 20 levels.
There are too many people who have let jobs and relationships slip away because they can't control the amount of time that they play. It really is too bad. I always thought, when I was a teenager, that when I 'grew up' I wouldn't be caught dead playing these games. But there are people who are retired, or people with families who play online to relax. Personally online games stress me out more than anything. I'm sure there are many who are able to play casually, and have a great time doing so as one of their many hobbies.
Maybe it depends on the game, and it depends on the person. I hear that WoW is for the pick up and play type of players, but I also know too many people who have given up jobs and their real lives to play that game 24/7. I wonder how people do manage to balance a real life and an online one?