From the introduction:
Anne McCaffrey's Dragonriders of Pern series remains one of fantasy's most enduring franchises, and the recent PS3 video game Lair allows the hero to take control of a mighty dragon steed. Young adult works like Eragon and How to Train Your Dragon follow young heroes as they bond with incredible beasts. Despite the popularity and sheer appeal of dragon-riding heroes, traditional fantasy gaming has never really supported the trope.
It's easy to see why: giving a player character a bonded dragon at first level is equivalent to giving that hero a fully armed tank, while everybody else in the party makes do with swords and bows. A veteran hero, riding an adult dragon... the metaphor of a tank gives way to an even more alarming one: the metaphor a fully armed stealth bomber with a nuclear payload. Dragons are overwhelmingly powerful, and having one in your corner, loyal only to you... that's a class feature that certainly outdoes Weapon Specialization and Barbarian Rage.
This sourcebook presents a new 20 level core class: the Dragonbound, a unique breed of knight who is defined by his empathic bond with a great dragon steed. The Dragonbound is an intentionally unbalanced campaign option; in terms of sheer damage dealing potential (as well as mobility and endurance), the Dragonbound certainly outclasses the Fighter and Barbarian. Character classes that depend on a mystical mount or animal companion, such as Paladin and the Ranger are similarly outclassed. Since the Dragonbound's core concept resolves around a partnership with a dragon, and mounts and animal companions are tangentially related to the concept of what a Paladin or Ranger is, the Dragonbound's companion beast is naturally more impressive. However, given the Dragonbound's emphasis on honorable combat and exalted place in their nation's military means that many Dragonbound eventually take at least a few levels in Paladin or Blackguard.