From The Blizzard World of Warcraft Patch Notes:
The Rest State system has undergone several changes. There are now two tiers of rest states: Rested = 200% experience and Normal = 100% experience.
At level one, players will all start in the normal tier. When a player rests be it in a city or at an inn they will gain rest bonus at a very slow rate. Eight hours of rest will be needed for a player to gain one "bubble" of rest bonus. At any given time, players will be able to accumulate a maximum of 30 "bubbles" worth of rest bonus which translates into approximately 1.5 levels worth of rested play (before your character returns to normal rest state).
The changes to the Rest State have been made with the understanding that most players will be in the Normal experience tier most of the time. The Rested tier is primarily intended for players that take a day or more off from playing or are very casual with their play schedules. This has enabled us to greatly reduce the experience necessary for all players to level regardless of which rest state tier they play under.