Some people don't want to worry about staying in character; they just want to come home, play a game and chill out. That's fine, they have the choice to be a regular player and do what they enjoy. But for those of us who seek the path of the roleplayer, we ought not to stop there.
We spend a lot of time in WoW doing all the same things other non-roleplayers are doing, whether it's questing, instances, or PvP. In the process, it can be easy to let one's character slowly drift away from a genuine personality, and into a mere avatar for your own personality as a gamer in a computer game. After all, your character must do a lot of things in order to progress, many of which are game-oriented goals rather than story-oriented goals. You need boss loot, Badges of Justice, Arena points and a bunch of other things that don't always translate well into very interesting character motivations.
It's easy to rely on old standby motivations so much that they become excuses. We might say, for example, "I'm trying to help the Shattered Sun Offensive to prevent Kil'Jaeden from entering Azeroth!" or "I'm hoping to attack Pathaleon the Calculator and take from him his prized sword: The Sun Eater!" And these are fine reasons for characters to do things, but we must remember, there's nothing really new or interesting about them. Every one wants to prevent disaster, or acquire new weapons -- but what about such a desire reveals who your character really is? How can you make normal gaming goals and activities into an opportunity for interesting performance and immersion in a fantasy world?