Assuming you have no issues with your game currently, I agree with previous posters, copy and paste
If you have an ext hdd, copy your entire wow folder from c:\users\public\games and drop it back on your new windows installation. Wow is ABOUT 30 GB give or take. Everything will be preserved, addons, configurations, preferences, etc. And its MUCH faster than having to redownload, install and patch the game, especially if you use multiple addons
Copying the game back and forth can take up to two hours, if not more, over USB.And its MUCH faster than having to redownload, install and patch the game, especially if you use multiple addons
Simply reinstalling the game to the point of playability will take 20 minutes.
~60gb over USB takes a number of hours, unless you're on USB3, AND have a USB3 drive. Which most people don't yet.
Backup of addons and wtf takes.... 25 seconds. Downloading wow takes 20 minutes tops, unless you're on dialup. Having a nice clean install of wow not only takes less time, but ensures that little bugs that simply repairing wont fix, are handled.
I'm using USB 2.0 on an external HDD. I get about 30 megabytes/s. For a 22 GB installation, that should take me about 12.5 minutes. You're probably thinking of a USB flash drive which has brutally slow read and write rates.
Have you ever actually tried that function? Playing before the game is fully downloaded is a humongous pain. You can't see anything, everything is blurry and its just an overall bad experience. I rather spend the 30 minutes copying WoW on and off my external hard drive. You can even play WoW straight off the external hard drive if you wanted to.
Copy your WoW folder to an external HDD. Can copy it about, it still runs wherever.