Blizzard's returning-player guide to WoW Forever
Blizzard's overview for returning players: WoW Forever is set in the early 'Year 1' era of the original 1-60 world, with more than 1,000 new quests, new zones like Mount Hyjal, Shen'dralas, the Riverglades and Zephras Isle, nine new five-player dungeons, new race-class combos (Human Hunter, Dwarf Shaman, Gnome Priest, Orc Mage, Troll Warlock, Undead Paladin), the Skyborne race (Heroic Pack or higher) and the account-wide Legacy System.
Source: Blizzard Entertainment, worldofwarcraft.blizzard.com