It's complete and utter escapism. A time sink. You will spend hours/days/months even years staring at a monitor ignoring everything else around you. It's designed to pull you in, light up the brains reward centers endlessly (new levels, gear, areas, spells etc.) encourages you to join in with other players and start interacting with them on a daily basis (online friends, guilds) and always after you complete one task, to have another tantalizing goal just on the horizon. It's remarkable how well it can hook people.
Like others have mentioned, there are plenty of cautionary tales. I played for about 2 and a half years, everything suffered as a result. My online character was growing and becoming really strong while in real life I had stopped working out, wasn't going out with friends as often and my grades in college were dreadful. It didn't help that I got 3 other friends to pick up the game too, so there was always a friend online. I think all of us got out a couple of years back after maxing out characters, but it was like a drug in the heyday, 9 hours of play a day wasn't unreasonable.
