Your character is like an arrow. He was launched from the birthplace of your imagination with the aim of creating spontaneous stories with other creative people. Your character's personality is the particular direction he travels in, and his background story is the bow which set him on his way.
The bow-string tension that gives a good backstory its momentum is its lack of resolution. The desire to find resolution propels your character forward into the game, but it doesn't predict with certainty where your he or she will end up. Realizing this can free you of a great burden: your story doesn't have to make the New York Times Bestseller List. In fact, the whole idea here is to purposely leave your backstory unfinished, ready to be resolved through roleplaying. Too much emphasis on a dramatic background leaves you with not enough room for an interesting foreground, and little else to contribute other than the saga of your epic past.
Obviously, people aren't logging into WoW to read your miniature novel. They generally won't want to hear your backstory unless they specifically ask you about it (which they might!), but even then they'll care less for its narrative value and more for its ultimate impact on your character as a person. It's best to think of it less as a story in itself (e.g. "How I got to be this way"), and more as a prologue to the story you want to roleplay (e.g. "How do I get out of this mess?"). Its purpose is to set up challenges for your character to overcome with other people, and it should establish a direct line to your character's desires and aspirations.
"The arrow that flies..."
One character might be an old man whose greatest wish is to pass on something to the next generation before he dies; his previous experiences are the forge that shaped his thinking and the various stories within his backstory may be among the basic tools with which he nurtures his wisdom in the hearts of others. Another character might start out as an eager explorer in search of adventure and glory; and in this case her backstory should be as simple and boring as possible, so as to give her all the momentum she can get thrusting forward into the future, away from that undesirable past. In both these cases, the backstory focuses on real human motivation and helps the character to explore a particular theme his or her player likes. (Note: Beware common backstory groaners -- even if they seem like a good idea at the time.)
Probably the most popular theme roleplayers like to deal with is that of abandonment and loss -- in fact this theme is so common that it often seems cliche or Mary-Suey. The fact is, however, that many of us experience a lot of loss and abandonment in our real lives, so it's natural that we should want to deal with it in our chosen art form as well. The problem comes when we let our characters' tragic lifetime of sorrow become a mere rut with which we dwell on such suffering, rather than a path through which they arise to transcend and make meaning of what they have gone through. A character whose backstory leads him to stubbornly insist that he has no enticing opportunity for growth and change has no place in a roleplaying community, because, in spite of a plethora of backstories he could tell, there's no more story for others to create with him together.
Characters based on the theme of loss often have to deal with the death of some family members. Those deaths are only a good thing for your character if they actually influence you make relationships with other people in the present. Acceptable influences might be: "I must find someone to help me find out who killed my parents," and "I must gather a group to avenge them!" Even better, however, would be something less like Hollywood and more like real human feelings, such as: "I know I will never have my parents back, but I still long for someone to share good wisdom with me, and help me to grow," or "The memory of my parents must never die! I must make up for their loss by achieving all the things they would have wanted, realizing their hopes and dreams through my actions."
"...the bow that is stable..."
Once you view your backstory as a kind of propeller driving you forward, then creating a good one becomes much easier. You may need to do lots of research on Warcraft lore, or pick a simple template with the emphasis on the story from level one on up. In fact, you may find that your character doesn't need any backstory at all -- the details you can make up on the spot as you wander about and interact with people may be perfectly suited for your needs.
Also, it's important to remember that much of your backstory may never actually be told -- certainly there will never be time in the game itself to type out pages worth of information and descriptions. J. R. R. Tolkien is famous for his immense backstory for The Lord of the Rings, the vast majority of which never got even a single mention in that classic novel. All of it influenced the story in different ways, but only those parts which were directly relevant had any place in the book. In the same way, only the details of your character which influence the way you roleplay him or her today are important -- the rest are just bonus special features you've got tucked away.
In addition, unless you wish to write out your stories on your RP guild's website forums, you can keep most of your story in a kind of personal limbo in your notebooks or in the back of your mind. The only things about it which become real or unchangeable are those details which come out in the natural course of roleplaying, either through direct storytelling or through clear implication. You are totally free to improve any aspect of your character's backstory as long is it doesn't conflict with what the people you currently roleplay with already know about you.
"...the mark upon the path of the infinite." -- Kahlil Gibran, The Prophet
"History repeats itself," or so they say, but however true that may or may not be in real life, in roleplaying it is resolutely false. We create our characters so that their futures may be different from their pasts -- so that the questions they raise from the back storehouses of our minds may find come into the light of creative companionship and find answers.