The dead zone references the distance in which a player cannot do any useful action against a mob. Generally this term is used by Hunters and Warriors to refer to the distance over 5 yards and less than 8 yards away from them. At 5 yards and less, a player can engage in melee combat. At 8 yards and more, a player can engage in ranged combat.
Between these two distances, a Hunter finds nearly all of his (or her) options unavailable other than Scatter Shot, Volley, and using their pet. This is no longer true after Patch 2.3. See below.
Similarly, a Warrior cannot use any of his melee attacks or Intercept abilities in the dead zone. Intimidating Shout and Demoralizing Shout are the only Warrior abilities that affect enemies in these ranges.
This dead zone can be exploited by mobs with rooting abilities. The mob can root the player and stay in the dead zone untouched during the duration of the root.