Every superhero has an origin story. Peter Parker was bitten by a radioactive spider that turned him into Spiderman; Kal-El was sent to Earth from a dying planet and became Superman; Bruce Wayne vowed vengeance when his parents were killed, and years later the Batman was born.
Back in my teens, the heyday of my comic-book reading, one of my favourite superheroes was Wolverine. He was one of the Uncanny X-Men, a team of mutants who, under the wing of Professor Charles Xavier, fought the bad guys on a regular basis. Wolverine remains a popular character, judging by the relaive success of the films and many comics that bear his name, but back then he was a bit of a mystery.
Was Logan his real name? Who had laced his bones with Adamantium? He was known as Weapon X, but what did that mean? All these questions have no doubt been answered by now, the character given a definitive origin story, but in his case is it really necessary to reveal all? I liked that he was this man of mystery, the secrets of his past merely hinted at, drip fed for the discerning reader to piece together.
That said, there's great benefit to telling the origin story. Not only does it give us an insight into the motivations of heroes - "Criminals are a superstitious and cowardly lot," says Bruce Wayne, "I shall become a bat!" - but also villains. Take Magneto for instance, who as a small boy suffered under the atrocities of Nazi Germany. Knowing this, we can understand the reasons for his actions; it gives the character depth, almost making him into an anti-hero rather than straightforward villain, and helps to ask the question "what would I do if I was him?". Magneto goes about things the wrong way - he is a villain, after all - but his goals are understandable given his past.
Done well, an origin story can be superb. Take Doctor Who and Genesis of the Daleks, in which we're shown how the Daleks were created; it's one of the finest stories in the show's entire 50 year run, producing many fine moments of drama, depth and emotion. On the other hand, there's the second series of Heroes; what started so brilliantly now becomes a convoluted mess as bad guys are revealed to be the brothers of good guys (or were they? Do you know, in all the chaos I really can't remember) and a conspiracy unfolds to epic proportions.
I'm worried about Doctor Who. Showrunner Steven Moffat has promised that, in this the fiftieth year of the show, the Doctor's biggest secret will be revealed. Given that he asks "Doctor Who?" a lot these days, I suspect we'll get to know the Doctor's real name. Question is, do we really need to? Isn't it enough that he's a renegade Time Lord who shunned his society to go on the run? I'm concerned we'll get to know too much of why the Doctor left Gallifrey (I have a horrible suspicion he'll turn out to be Rassilon) and - like the creation of Darth Vader - it's something that's best left to our own fertile imaginations. Then again, we were never shown a regeneration as the start of the 'new' who, so could there be another Doctor between McGann and Eccleston, so could there be someone else in between that we don't know about? All pure speculation, of course, but that's the beauty of not knowing - we can theorise to our hearts content; it gives us something to talk about, other than having a like/dislike opinion.