Minecraft wiki entry for Exploration_Update, associated text: Entity IDs, block IDs, block entity IDs, and item IDs are no longer case sensitive. Plus, some names have had underscores added or have been completely rewritten.
Lots of changes to NBT tags.
Chat messages are now 256 characters long.
Various items, such as signs and wooden doors, can now be used as fuel for a furnace.
Shields now block 100% of damage dealt in melee combat.