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

Quests

•  [70] To Netherwing Ledge!, Part 4 of the Netherwing quest chain, which starts from Mordenai in Netherwing Fields.
•  [70] The Missing Magistrix (Isle of Quel'Danas / Throne of Kil'Jaeden)
•  [80] Aces High!, A daily quest in Coldarra that acts as training and practice for the final phase of the Malygos raid in the Eye of Eternity
• 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