Blizzard has explained how the WoW Forever Legacy System will work when the game launches. It gives players long-term goals for leveling, professions, exploration, PvP, dungeons, and raids. Completing those goals earns Legacy Points shared across a Battle.net account, while each character chooses separately which perks to buy.
The system opens as soon as you earn your first point. For many players, that will happen at level 25. Reaching 150 in a non-gathering primary profession or exploring the entire world map can also unlock it. Blizzard lists 65 launch challenges in six categories, and each completed challenge awards one point.
At launch, each character can spend up to 16 Legacy Points across three perk trees: Professions, Adventure, and Resourcefulness. The points are shared across your account, and every character has its own perk choices. Blizzard says a single character can earn enough points to reach the cap, so leveling alts is optional. Players who keep completing challenges after hitting the cap still make progress toward account-wide rewards.
The perks cover different parts of the journey. Well Rested raises the rested-experience cap and helps it build faster, while Frequent Flier cuts flight costs and speeds up travel. Profession options can improve skill gains or gathering returns. Resourcefulness perks can reduce death-related durability loss, extend buffs, or remove the need to buy reagents for certain class abilities.
The launch reward track includes a Spectral Bear Cub pet, a Spectral Bear tabard, a Spectral Bear mount, and a Replica Ironforge Air Rifle toy. Blizzard plans to add more challenges, perks, and rewards with future content updates. Hardcore will arrive later with separate Legacy rules; progress earned in other modes will not transfer into Hardcore.
Source: Blizzard's Legacy System guide. Image: WoW Forever Legacy rewards screen, © Blizzard Entertainment.

