The ESRB, the ratings software board in the US, has officially released the ratings for Call of Duty: Modern Warfare. Call of Duty: Modern Warfare was officially rated M for MATURE with Blood and Gore, Intense Violence, Strong Language, Suggestive Themes, Use of Drugs, In-Game Content Purchases, and Users Interact. But beyond that overall rating, they have also posted a rating description.
Source: N4G PC Call of Duty: Modern Warfare ESRB Description revealed