I'm not really afraid that, I guess?
I rarely share my story-ideas in detail, but that's less out of fear of theft, and more because I don't want to spoil the events of the story, in case I ever get around to telling it.
Also, a story changes a lot from initial idea to finished story - by the time I start posting pages, it might be nearly unrecogniseable from that first spark of an idea, so it's not like things are immutable and permanent either. And like Shazzbaa said, there are more ways to tell a story than just one. There are as many ways as there are storytellers.
Just think about it: "young hero must leave the comforts of home to go on an adventure and defeat evil" could describe an infinite number of existing stories, but not one of them is identical to another. Lord of the Rings is not the same as The Belgariad, but they can both fit into that single sentence description.