Fog Begone!

Fog is a dastardly thing, and its removal is not provided for in any graphics options screens.

Fortunately, there is a way to remove it for good and reclaim one's sight and a couple points of FPS.

STEP 1. Go to the WTF folder. This is usually located at:
C:\Program Files\World of Warcraft\WTF\

STEP 2. Open the file "Config.wtf" with Notepad. If you're worried about altering it accidentally so it breaks the game, save a copy of it first.

STEP 3. Append the following code to the end of the file, save and exit Notepad.

Voila!

Source: http://www.wow.com/2009/10/13/turning-off-the-fog-effect/