Now if this sounds crazy, hear me out:
I started a human priest, leveled her to 13, decided I sucked and quit. I have quickly leveled an undead shadow priest to 29, and have stopped leveling (despite great success grinding using advice posted here) in order to run BGs and get some PvP rewards before continuing.
Sometimes, it can be difficult getting BGs together, and because I enjoy playing a priest so much (especially running instances), would it be a bad idea to use one priest from each faction and level them, depending on the number of players playing at the time?
For example, if my 29 can't wait 19 minutes for the next BG, switch over to my 13 and (now that I know what I'm doing) continue leveling her.