World of Warcraft's shiny new tech will allow you to group with players from other servers in the open world. Cross-server zones, going live on the Mists of Pandaria beta servers shortly, are meant to alleviate problems with low-level zones being too underpopulated for newbies and alt-levelers to find groups.
The tech will create phased versions of the zones and throwing players from multiple servers into them. When you enter a zone flagged as cross-server, you'll be placed into the phase seamlessly, just like you would moving between story-phased areas elsewhere in the game.
The creation of these cross-server phases will be dynamic, and based on which zones are deemed underpopulated at a given time. The rules for looting and trading will carry the same restrictions as if you were in a dungeon or raid with players from multiple servers, to protect each server's individual economy.
By default, you'll only be mixed in with players from a "select group" of realms which will match your server type (PvE, PvP, RP-PvE, RP-PvP). Leaving a cross-server phased zone for one that is not currently cross-server will not disband your group, but you will no longer be able to see or interact with anyone in the group who is not from your home realm.
One of the best bits of news is that this feature will allow you to group with Real ID friends who play on different servers in the open world. It'll work like the existing cross-realm dungeons, battlegrounds and raid functionality: just right-click on your friends list and party up. When friends are coming from servers with different rulesets, the party leader's server type will be the one that everyone joins.
Blizzard also notes that this technology can also be used to create multiple phases of a zone on the same server, a trick that many MMOs nowadays use, but WoW itself has never done.
Check out the WoW blog for an in-depth FAQ with all the details.