Simple. Just think it would be a nice read to see everyone's most memorable experience in a short story about each expansion, or the thing that you would most remember the expansion with, or just what you consider was the key thing that makes each expansion unique and different (better) than the others. Include in that Vanilla and MOP.
My story isn't very long. I stopped playing after the end of Wotlk, but even years later I keep checking these forums and news about the World of Savagecraft.
One of the things most memorable for me during the Burning Crusade actually comes from the cinematic. Seeing the draenei paladin all armored and casting seals out of the libram in the old sanctuary building decided the way I'd spend my years of playtime in Azeroth. I fell in love with it, and rolled a draenei paladin. My goal was simple: do everything to one day get my hands on the emblematic, unique and absolutely amazing Hammer of the Naaru. And I did it :P. Might share other things later on if the thread gets lively.
From Wotlk what has stood out for me as an experience is the unpleasant, but yet remarkable, encounter with Syndragosa. You see, I played on a Pentium IV pc that rocked the world in it's prime, but in that boss fight, in the suck in phase, I had no way to move out with 1.5 fps, but to put the 320x640 resolution (the lowest one) with which you can't read the chat or see anything clearly. All that in the sake of being a top end raiding Ret paladin, culminating with the 10man achievement ICC drakes. A remarkable memory in any case.
Share yours!