There was an interesting discussion that happened on the official forums the other day, and I think it deserves a bit of light here on WoW Insider. Of course that thread devolved quickly into standard forum QQ and blue bashing, but hopefully that doesn't happen here.
The thread asked: are achievements optional?
This is a loaded question, for sure, so what I'm going to do is look at three different types of players and analyze what achievements mean for each of them. But of course to begin, it should just be said that any achievement is completely optional in the game. You don't need them, they deliver nothing to you that's necessary for your character to progress.
The casual is the person who chooses to play the game at their own pace for a little or as much as they want. The casual rarely, if ever, make appointments or schedule in game activities with other people. The casual may or may not be in a guild, and that's just fine. The casual is there to enjoy the game on their own terms.