In WoW there are two ways you can turn your character. You can use the keys you have bound to spinning left/right, or you can use the mouse. The mouse has no maximum spin speed, whereas the keys only spin the same speed every time. A player who is using the mouse will, as a result, be able to get behind a player using the keys. A whole host of bad things happen when your opponents can get behind you reliably (You can't dodge, block, or parry, some spells have facing requirements, etc). Its also just much worse in general because of the speed difference. Generally, if you are key board turning you are also clicking your abilities with the mouse rather using keys, which in turn is also slower. So, in essence, keyboard turning makes you less agile and slower to react, and the difference is quite noticable to those who know what to look for.
Just think of a keyboard turner as WoW's version of a mouth breather and you're pretty much there.