Blizzard are bellowing the forge in readiness for Mists of Pandaria, and have released patch 5.0.4 to start laying the groundwork. One of the major talking points is the addition of account-wide achievements, pets, and mounts, allowing you to share the majority of them from one character to another.
The other major item on the list are new cross-realm zones, where you can form a group with players from other realms while inside the zone as if they were on your own server. This functionality will be rolled out to all servers once Mists of Pandaria launches, but is currently only available on select servers.
Read the full patch notes at Battle.net.