New patch download?

It's not a new patch, it's actually "re-downloading" data which will replace current files. It's intended to resolve a few issues including disconnects and 'pink squares'.

As it's not a patch, if you're extremely eager to get into the game (or it's taking too long) you can actually bypass the launcher and run the wow.exe in your game directory and play normally. You can then continue on downloading it later after you're done playing.