Minecraft wiki entry for Development_versions, associated text: Enchantment system has been rebalanced.
The maximum enchantment level has been lowered from 50 to 30.
As a result enchantment tables now require only 15 bookcases around them to allow maximum level enchantments.
With a full enchantment table set with bookshelves, at the bottom of the 3 enchantments, it always shows the player the highest number possible.
Using level 30 enchantments, the player has a lot more luck; almost all items get at least 2 enchantments.
Various actions will now reward the player with experience orbs, such as mining ores (except gold ore and iron ore), using furnaces, and getting achievements.
Levels are now awarded linearly. It takes 17 experience to go up one level.