It would have never even have occurred to me that this might be an issue for gamers out there, but apparently it is. Casting a spell while moving should feel like breathing for any serious World of Warcraft player. Are you serious?

Moving should not stop you from casting in World of Warcraft. In fact logically, a person should be able to move and do things at the same time. In game though a simple move forward or a jump upward will almost always interrupt someone mid action. This mechanic has been in MMOs for as long as those swords named Masumune and Excalibur existed. -- About.com Video Game Cheats' Contributing Writer Martene Battle.

via Abusing Spell Casts While Moving in World of Warcraft. Read the previous link for the entire article.