From Vanilla to Wrath, my only character above level 20 was my main Druid. I started Tauren Hunter but stopped at 20 and started my Night Elf druid, it stuck and never looked back. In BC I started a Draenei Shaman but I only got her to level 50 by the end of Wrath, I was so focused on my Druid. I started leveling side characters in Wrath and in Cataclysm I had a whole menagerie of them, but my Druid was still my main.
In MoP I leveled up a monk and she became my main.