The New World of Warcraft: Cataclysm Launcher allows you to stream game data while you play. This means less time waiting for patches, and more time playing.

There are three stages of download progress:

Core game content is being prepared. You cannot enter the game at this time.
Major game content is not completely applied. You can play, however your game experience will not be ideal.
Final game content is almost complete. You may experience a few game issues while playing at this stage.

If you find the play experience undesirable you may exit the game and resume this download.