World of Warcraft has received a major patch in anticipation of the new expansion, Mists of Pandaria.
The main points of the patch include account-wide achievements, pets, and mounts; there's a useful FAQ on how to bring your stuff to all your alts rather than just the character that earned them.
A new world event will go live in the weeks leading up to Mists of Pandaria's September 25 launch. Attack on Theramore Isle involves an attack on Theramore by Warchief Garrosh. A nice little addition is area-of-effect looting, so you won't have to check every single corpse around you, and Battle Tag information will be available in game for those who choose to enable it.
Perhaps most interestingly, the patch introduced cross-realm zones, which should help you find group mates in less populous zones ion your home realm. It's only available on some realms at the moment but will spread over the next few weeks.
There's some news for Mac users, too. If you're still on OSX 10.5, be aware that you're no longer supported - although seriously dude, time to upgrade. Blizzard has added support for Retina displays and weird Mac resolutions, and tweaked the client to make it more usable.
And then of course there are loads of nitty-gritty details on tweaks to the user interface, classes in general and in specific, items and professions. Read the patch notes in full on Battlenet.