Content exclusive to flying mounts

Certain areas of World of Warcraft can only be accessed on or by using flying mounts.

Quests

 * To Netherwing Ledge!, Part 4 of the Netherwing quest chain, which starts from Mordenai in Netherwing Fields.
 * The Missing Magistrix (Isle of Quel'Danas / Throne of Kil'Jaeden)
 * Many quests in Icecrown and Storm Peaks

Areas
These areas can be visited if you get a warlock to summon you.
 * Throne of Kil'jaeden in Hellfire Peninsula
 * Netherwing Ledge in Shadowmoon Valley
 * Elemental Plateau in Nagrand
 * The Barrier Hills, Skethyl Mountains, Blackwind Valley, Blackwind Lake and Skettis in Terokkar Forest
 * Blade's Edge Mountains Plateaus in Blade's Edge Mountains
 * A large portion of Storm Peaks
 * A large portion of Icecrown
 * Some areas of Sholazar Basin

Instances

 * The Mechanar in Tempest Keep, Netherstorm
 * The Botanica in Tempest Keep, Netherstorm
 * The Arcatraz in Tempest Keep, Netherstorm
 * The Eye in Tempest Keep, Netherstorm

Starting with patch 2.4.0 it is possible to be summoned by a warlock into the above instances.
 * Naxxramas in Dragonblight
 * Halls of Stone in Storm Peaks
 * Halls of Lightning in Storm Peaks
 * Ulduar in Storm Peaks

The above instances/raids were added in Wrath of the Lich King and patch 3.1.0