Many things that are harmful in large doses but beneficial in moderation. Alcohol has many beneficial effects when used in moderation, but too much is both immediately a problem and can cause systemic problems. Drugs (of the legal/medical sort) are specifically used because of their beneficial nature, but most of them cause harm when used in excess.

The problems come when something has a feedback loop which promotes more usage beyond normal usage. Alcohol has this, and so does heroin. Different games have this to different degrees. Like most things I've mentioned, different people are susceptible to this to different degrees, which complicates the issue.