Bungie has published a list of everything included in that "we plan to prove that The Taken King is more than DLC--it’s a game."

A list of content included in the DLC, as written by Bungie, is pasted below.

• New story missions and an array of sidequests that conspire to tell the story of our battle to defend our Solar System against Oryx and his Taken army
• New narrative driven by cinematic cutscenes starring an engaging cast of characters
• New Exotic Weapons, Exotic Armor, and Exotic Questlines
• The Dreadnaught, a full new destination, with unique secrets, rituals, and treasures to uncover
• A new breed of enemy combatants, The Taken, armed with new abilities and weapons
• An increased Level Cap, and new ways for your Guardian to grow more powerful
• The most substantial injection of new, unique, and redesigned weapons, armor, and talents to date
• New subclasses with all new Supers and abilities
• New cooperative Strikes with unique and dynamic new Boss battles
• Redesigned and reimagined Strikes – Dust Palace, Undying Mind, and Cerberus Vae III – are now overrun by the Taken
• New Crucible multiplayer maps with new Crucible modes, including the previously revealed Rift and Mayhem
• And, of course, last but not least, our biggest six-player Raid yet.

While these promises regarding content have been made now, Bungie says in the weeks ahead it will also discus "a range of topics that we know are important to you, including weapon rebalancing, economy improvements, an overhauled quest experience, bounty enhancements, faction allegiances, end game content, and more."