Shakespeare is talking about the World of Earth here, of course, not the World of Warcraft. By "parts" he means the various stages of your life, in which people understand themselves differently and play different roles in their societies.
There's a sense in his words that the movement of people though the world happens according to a certain script, and that the script itself, the roles people play, are all just a facade covering up the truth, hiding the true nature of each "player" in the world. Yet each person can bring the truth within themselves into the art of playing the roles we're given in life. One could argue that all the best "players" of life are the ones who play whatever role they choose with all their heart and soul -- never just going through the motions of life because we're too bored to do anything else.
Roleplaying is a way of putting your heart and soul into your game as well, rather than just using the game to pass the time. You might say that all of WoW is a stage, and we are all merely players: we have our logins and our logouts, and one man in his time plays many alts. The actual the game itself, if we play it out of boredom, is mere entertainment. But by investing a bit of our personal energy into the experience and the community, we can find a greater meaning in it as well. After all, if we choose to spend our real life hours playing WoW, it makes sense to do so in a way that makes it something meaningful and worthwhile.
However, not everyone who wants to live his life and play his games with that certain artistic passion need be a roleplayer. WoW can be a different sort of stage for everyone who plays it. For some, it's a sports arena, a field for them to compete against other players, acquire their rewards and bask in the glory of victory. For others, it's a stage for cooperative challenges in raids and adventures that require people to accomplish something by working together. The only difference between these "normal" players and roleplayers is that we do either or both those things with an imaginative frosting added on top: a stage on which our imaginary characters to live and interact with one another.
In South Park, the ultimate WoW player was depicted as "one who has no life," who can spend all day and all night leveling or grinding or even raiding monotonously. But to a roleplayer, the ultimate player is one who is so full of life that it spills over into everything he does, even his favorite online multiplayer computer game. Like any true artist, he brings to his interaction with others something that they can benefit from, either through inspiration, humor, or some other reflection of the human experience. This ultimate player's status comes not from his epic loot or his leet PvP ranking, but from the pure joy of others in spending time around him, whether or not he actually "roleplays."
All of us who play WoW are members of a community, players on a stage, and our characters are the costumes we wear. Every time we log on, each of us has a special chance to use these costumes to interact with other players in some way that will make them smile. Perhaps your role will be a kindly tauren scholar with adorable glasses that keep sliding down your muzzle, or perhaps you just share your day's experiences with your guildmates and go have fun together. Perhaps you are a Gnomish warlock with a master plan to take over the world just as soon as you can summon your army of killer demon-bunnies; or perhaps you just want to lead your team to victory on the battlefield and have a good time doing it.
Whatever role you choose to play, the important thing is to cherish the moments of your life you spend playing, and respect others who do the same.
Now that you've read this far -- I'd like to hear from you about what sorts of roles you play on your stage in the World of Warcraft.
Are you a roleplayer? What's your character like? How do you express yourself creatively through him or her? How do others respond to this expression?
Are you not a roleplayer? In what other way do you put some of your heart and soul into the game so that your fellow teammates are somehow benefitted by your being there? How do you view the role you play in the game?