Not only that but I am rather fond of my ideas and stories and don't look forward to having them ripped off or stolen by some passer through. It is not the main users of the site that I fear/don't trust. It is the public nature of any person being able to access my hard work and swiping it for their own purposes.
I can see the point of not wanting yourself "published" until you actually send it somewhere where it can be published in the official/traditional sense of the word. But we have one member on the site that regularly participates in our writing challenges, posts his stories and then turns around and sells them to someone where they will be published somewhere else. What he posts for the challenges is just a rough draft so he works on the stories until he gets it as close to perfect as he can before sending it out. There are some publishers that are sticklers for that "has anyone seen this yet" type of publishing and some who don't care. I say read their guidelines and know your markets if you have any doubts.
As for the second fear does it matter? If you post something online there is a posting stamp that gets attached to whatever you put up on the net (whether you can see it or not). People will know exactly when and where you put something out there. As far as someone stealing your ideas and claiming it as their own they'll have to prove that they came up with the idea before that date of your posting. Believe me that's a much harder thing to do.
As for someone taking your idea and being the first to get it out there in the traditional publishing world well, why aren't you doing that yourself? If you've got an idea that you think is so great and special why aren't you taking the time to get that idea out there? Write. Submit. Publish. Too many beginning writers have the egos that think they've written the next great American novel and the paranoia to think that everyone is out to steal it. I hate to tell you this but I'm pretty sure all the stories ideas and combinations of what could happen in a story have already been done before. You're not going to really write anything new. You'll just have a different spin on things. And when you think about it only you can put your unique spin on things. If someone else took the same basic idea their going to have a completely different story on their hands than what you might have done.
If you want to write and get feedback from some kind of writing group but don't have the means in your neck of the woods the internet is a good place. Just look around for websites for writers that offer private writing groups so that when you post something for people to look at the only ones who see it are those in the group. It would be no different than if you passed out a piece of paper to group that met in the library once a month.
Most of all know your markets. Know what publishers will accept and what they won't. If you have a story that has been "published" on the internet in some forum or another and a publisher won't take that type of writing, write something else for them that hasn't been published. Above all don't' be paranoid. Fear gives in to procrastination and writer's block. Don't put the cart in front of the horse to use a stale old cliche. If you spend your time worrying about if the horse is going to throw a shoe or if the cart is going to break a wheel instead of getting out and starting the journey you're never going to get there. Be brave. And keep writing.