#11 - Sept. 29, 2006, 4:32 a.m.
Just to "be puttin' some official all over this"; all current realms will run expansion content and the only thing restricting players from that content will be if they do not own the expansion. Those that buy the expansion will be able to access the new areas, such as Outland, the Draenei and Bloodelf starting zones, and places in the current landmass such as Karazhan and The Caverns of Time among other features only available through those new zones.
Some content will apply to all players, such as the new honor system, new talents (the expansion is needed to level to 70 though), the new LFG system, and any other general changes that affect the game overall and are not specific to the expansion content.