World of Warcraft wasn't really designed to be played in first person. First person was put in though so that people could get the full effect of the scenery around them and experience it but the mechanics of it aren't as optional as third person.
I used to play only first person in a lot of games (minus fps' which should be in first person :) ) and now I don't know how I did it, especially when it came to PvP. It's just better to be able to see around you and behind you in third person so you can react to what is happening.