I imagine many of our first mains happened to be whatever class we had fun leveling the most. If you were in beta perhaps you had a better idea which class you wanted to play. I remember hearing Warrior was the overall easiest to play for a beginner so that was my first choice and subsequently first main.
Playing a warrior unwillingly corralled me into being a leader and as a follower by nature this didn't sit well for me or the groups I tanked for. My next choice was priest, the premier healer at the time if anyone remembers those days. I stuck with it for quite a bit. It became my main and for a while I couldn't fathom playing anything else, other than piddling around on alts. Raid healing lead to burnout and in came my druid which I dpsed and even tanked some before coming back around to healing.
Now usually players who have several alts all around the same level have this decision to make. And with each expansion some of us have taken the opportunity to swap main, most importantly to get a spot in the new raids released.
With the introduction of achievements you can become pigeon-holed into a main. I know I don't even want to consider getting an achievement on any alts. I can't imagine retiring my druid to start playing a new character and start trying to get the same achievements on it.
I remember reading a couple's blog where the tank had the legendary thunderfury. They both got tired of playing and needed the cash, so they sold both of their characters on ebay. It seemed surprising at the time. (Maybe only an addict would think so!) I would think a thunderfury would carry with it some emotional attachment and achievements are the same way.
The majority of players can agree there is very little focus on the RPG in an MMORPG. But achievements help to flesh out our characters. Like the Deed Log in Lord of the Rings Online and the Tome of Knowledge in Warhammer Online, your achievements tell a story of what you've done and in many cases what you've found are most interested in.
An interest in alts, unless you have a load of free time, is the only thing achievements won't capture.
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