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!