The two main changes have to do with the calendar system and the mount / vanity pet interface. You can take a look after the break for the exact text, and also keep an eye out on WoW Insider for the latest on both of these new interfaces.
One interesting thing to note: Elizabeth Harper is reporting that the mount interface isn't working. If you have a mount in your inventory, you cannot successful "train" it into your spell book, which means you can't cast or use it. This means those in the beta can't mount unless they already have their mount as a spell. An interesting tid bit that we'll keep you updated on.
Updated July 29th, 2008 at 5:35 p.m. EDT: Elizabeth Harper has reported that the mounts are now fixed in the beta. Mount up!
New Calendar feature: Players will be able to access the new Calendar feature by clicking on the new icon located in the top right corner of their mini map. The calendar will track in game events, allow raid scheduling, and more.
New Vanity Pet and Mount changes: Players will now be able to access their non-combat pets and mounts by clicking on the pets tab in the Character Info panel. These pets and mounts will be now learnable much like spells or abilities. Once a vanity pet or mount is learned, the icon will disappear from within a player's inventory and reappear in the appropriate tab under the pet tab under Character Info. Say goodbye to your vanity pets and mounts taking up bag and bank slots!