The expansion will add a brand new continent, like new quests and items and give seasoned heroes a brand new type of character to play, the Death Knight. In an informal survey of a dozen or so WoW players that I know, all of them were planning on picking up Wrath within a week of its introduction -- with a number of them (including myself) -- picking it up at midnight.
Wrath of the Lich King sells for roughly US$40 and should provide a significant amount of cash to Blizzard. The company tallies more than 11 million worldwide subscribers, though not all 11 million will front $40 because of how players are charged in Asia.
The prior expansion, the Burning Crusade, within 24 hours of release in January 2007, making it the fastest selling game "across North America and Europe." Expect Wrath, with WoW's larger subscriber base this time around, to do even better.