The new PTR (Public Test Realm) is now active (well other than crashes and reloads as they discover things are not going as planned).
Any hard core play should tranfer one of their favorite characters there as soon as they can. The changes are massive.
1) Talent Trees COMPLETELY change
2) Talent points GREATLY reduced. 71 down to 41?
3) Specs eliminated or greatly changes. For instance, Defense for tanks no longer exists. The defense stats have been converted to Dodge for tanks. Only Blood specd Death Knights (DK) can tank. Blood Presence is the tank presence now.
4) Spells and abilities changed or gone.
The current plan is for the changes to be rolled out in a patch prior to Cataclysm. This is a good thing for the players. It will give the loyal crown a chance to experience the changes.