You've raided Icecrown Citadel and killed Arthas. You've got a ranked arena team. Your gear score and achievement score both push 6,000. You're officially elite. Then one day that level 15 gnome rogue you rolled two years ago on a whim starts to call your name. She's got pink pig-tails and the cutest little laugh. Before you know it your little gnome is questing in Outland. Soon you're in Northrend. All of a sudden you're running heroics, and raiding. Now your little gnome is just as leet as your old main.
Back long ago I started a druid for the sole purpose of making leather kits for my guild. At the time I didn't realize that since my main was an enchanter, I made myself redundant because leather kits overwrite enchantments -- but I must have liked my druid. As I leveled that enchanter, a mage, my druid was never far behind. Now both toons are level 80, geared, and at the top of their professions. I honestly don't know which one of them is my main and which is my alt. The only difference is, as a healer, the druid has more utility in raids and heroics.
Have you ever switched mains? What would cause you to switch? Guild needs? Personal preferences? Switching classes? With paid server and faction transfers, a lot of the old reasons for switching toons have gone by the wayside. We want to hear your main switching stories.