We've done the quests. Hopefully we have read them as well. We should know the history, at least up to the point that we began. For example, I know the history of WoW for about four years back, perhaps three and a half due to leveling, so I know the world after the changes to Silithus. Others will know before and can say what Silithus once was (from what I've heard, a pile of sand with pretty much no quests and no interesting NPCs; a loreless wasteland).
We must not just remember this, but preserve it. I won't say we should write a history book. Someone should, but it wouldn't be me and it would be hypocritical for me to call for others to do what I will not. But let us gradually write our own histories, when we started, what we did, what we saw, what we experienced. Don't just talk about the NPC-based history, talk about the players as well. What sort of gear did we expect to start MC? How about a month later? What were the memes of the time? How has the community changed? If possible, try to avoid terms like better or worse, but do not stray from potentially negative terms; so if you see the community (or at least your server) becoming more elitist, say so but also provide examples.
Individually we will not have a complete history, but collectively, we might have the world as we knew it. When the cataclysm comes, it will destroy our world, but let us not destroy our history as well. I am glad to see in my previous post's comments that Sojourner has begun the quest.
Over the next few weeks I will begin to chronicle my own history. To avoid boring you excessively, I will try to make it sound more interesting than it was: "I farmed Twilight Cultists for a week straight" would become, "Seeking to bolster my reputation with the Cenarion Circle, I went to battle with the Twilight Cultists of Silithus, looting their corpses for the texts which are so valuable to those who worship the Old Gods, and those who seek to stop them." I apologize in advance if the 'plot' seems convoluted; I have a very distorted sense of time (for example, I still feel like WotLK is a couple months old and that Ulduar is a few weeks old at most, it gets worse as I go further back; BC is only a year old) and my constant alting destroys any hope of a coherent story.