In the vein of Alfred Hitchcock's Psycho (or even the pilot of Oz), they planned to pull the rug out from under the audience by killing Jack midway through the first episode, forcing Kate to take charge. After this sudden demise, viewers would realize no one was safe. [Damon] Lindelof says Steve McPherson, then the head of the ABC studio, made a convincing counter-argument that it would teach viewers not to trust the show, and the writers ultimately agreed with him.

In the end, it was decided that the character of Kate wasn't dynamic enough to lead the show; in fact, she had not originally been written as the fugitive she turned out to be. Instead, she was one half of a couple who had been separated in the plane crash. (Those roles ultimately switched over to Rose and Bernard.) Switching from Kate to Jack may have been the best course if her character hadn't been completely decided on. But it's kind of a bummer, especially when you consider that the show's creator, J.J. Abrams, had such great luck with his other two female-led shows, Felicity and Alias.

Photo credit: Tumblr

Who'd have been Lost's leader if they'd killed Jack in the pilot [Blastr]