At the early stages of concept and development for any story, all writers go through a series of doubts, queries and choices. In plot or summary form, the writer may not be wholly convinced that the story is taking its best shape, or whether the characters are working to their full effect, or if the pace and structure is smooth and efficient. Often, it is difficult to see the woods from the trees as the writer feels too 'close' to the whole affair and doubts if the story possesses enough positive qualities to take it through to script stage.
To avoid this uncertainty, sometimes it's worth going to first draft script immediately. It's easier to see the flaws and weaker areas of a story when it's down in script format, as opposed to having it laid out in a detailed outline/plot. Avoiding the tough demands of trying to figure out what's working, and what's needed, and what's not, is a natural impulse in the storytelling process. However, a basic passion about the concept and characters should be enough to convince the writer that the story is worth telling, and that while it may be hard work to get the story into shape, it's going to be worth it in the end.
So, here are a few questions that might help in getting over any potential writer's block: what was it about the concept that got you excited about the project in the first place? Is it a story you're burning to tell? Do the characters have an emotional and credible weight which makes you care passionately about them? In other words, do I respect them? Or is it more about plot - action/adventure/comedy etc?
Once you know the basic reasons why you want to tell the story, writing the outline or script becomes a somewhat easier task because instead of veering off into tangents about character and plot, you can continuously remind yourself WHY you're writing this story, and keep focused on what MUST happen (i.e. the plot elements that excite and interest you most, whether it's a character twist, an emotional journey or a big explosion).
Stick to your guns, so to speak. Knowledge is power, and while nagging doubt is part and parcel of the writer's process, it can easily be hurdled with a determined focus about the concept, characters and plot. This doesn't mean that you know the story inside-out; far from it, the characters and story may surprise you as you scribble it all down (a character may do something unexpected, leading you to a more exciting area of plot etc).
The point is not to get bogged down with the worry that the story's not worth it, or feeling insecure about the story's true value before you proceed. Remember the passion and excitement that got you this far, and hold on to that as you battle through the tough creative decisions that need to be made. Script readers and editors will have an opinion, and offer their advice, but they don't have all (or any) of the answers. It's all down to you.
Whether you're prepared to see the story right through to the end, or if you discover that the project's not got enough legs or merit for what you imagined, then it's all part of the writing procedure. Ideas come and go, stories last forever. Find the basic building blocks of your story (e.g. concept and characters) and go with your instinct. Entertain yourself. Let the characters surprise you but most of all, enjoy it.