I was in college (can't remember which year, probably my junior year) and my roommates and I were all hooked on Diablo II. So were our friends across the hall. We'd often play together as a group on the LAN, but we also enjoyed playing independently. We all played on headphones so as to not have the sounds of several simultaneous games clashing.
One afternoon, we were all playing simultaneously but independently. I decided to take a break, paused my game, removed my headphones, and stood up. Then I heard it: the sounds of rapid clicking coming from my roommates' mouses (and the mouses of the guys across the hall--we usually kept our doors open). I had decided to take a break right then because I had just leveled up, and that fact combined with the sound of the clicking gave me a moment of clarity: the whole game consisted of me clicking a mouse as fast as I could in order to make an arbitrary number (experience points) go up. Everything else is just embellishment. The fundamentals are: you click the mouse, the number goes up.
I felt like one of those lab rats or monkeys that has two buttons: one that delivers a serving of food, and another that stimulates its pleasure centers. They starve to death because they neglect the food button in favor of the pleasure button. Except I was worse, in a way: I wasn't getting a jolt to my pleasure centers, I was just watching a meaningless number get higher. Of course, I wasn't completely neglecting my well-being, either, but I was wasting a lot of time that could have been spent on more rewarding pursuits.
I uninstalled the game immediately and gave it away as fast as I could. I didn't give up video games, because not all games are such complete wastes of time, but ever since then I have set the bar very high for any game I play (similar to the post by snprbob86 about strict selection criteria). I'm especially watchful for signs that a game is just an exercise in "repeat simple task, increment arbitrary number."