First of all, no, this isn't a post telling you to just shut up. As far as I'm concerned, feel free to share your story ideas. I actually rather like the ideas shared in this article which inspired me to write this in the first place. In fact for most of you this article probably won't effect you at all. What this post is trying to do is inform you that there are actual significant legal consequences to sharing your story ideas online that prevent you from regaining creative or monetary control over your intellectual properties (that is, if you care about that thing in the first place).
The Copyright Act of 1790 reprinted in the Colombia Centinel above, image from Wikipedia
First Publication Rights
The importance of First Publication Rights is debatable in the Internet Age, but most people still treat it as very important, especially when it comes to IP monetization (and creative control). First Publication Rights is more or less exactly what it sounds like - who has the rights to publish a work first. As with Copyrights, First Publication Rights always extends from the author or creator of the work. There's always self-publishing (which has been around for as long as there had been written works) but traditionally it's considered much more desirable to transfer First Publication Rights to a "big house" publisher.
The whole thing about "self-publishing" is that it's very loosely defined, especially by the people who care about it most - the "big house" publishers. Even posting a very basic outline on a message board could constitute a complete "self-publication" of a work.
The reason why this is important is because these "big house" publishers aren't very keen on publishing something that has already "surrendered" its First Publication Rights. Of course, this is changing very rapidly - but the general advice from book publishing agents is to play it safe and protect your First Publication Rights as much as you can.
Protecting Your IPs
The second concern is protecting your IP from outright theft. You can argue this isn't a really big concern (unless you're very paranoid) but it's something worth considering either way. It's not exactly difficult to prove that an IP is yours when it exists mostly as a collection of words on a message board post with a cryptic or practically anonymous handle as the accredited author, but it's not exactly convenient either.
It also does nothing to protect you from what's in essence legal theft. If you've ever had extensive works published on, say, deviantART and had a hard time controlling those works popping up on other places, you know exactly what I mean. It becomes perfectly OK for random people to use those works under a very broad "fair use" clause. When it comes to images and visual art, it's not really that much of a problem and to call it "legal theft" is needless to say an exaggeration (which I put in just or emphasis) - in fact many artists count on or depend on people spreading their work through Fair Use Agreements. When it comes to an original story however, it can become more problematic depending on the intent of the author (who in many cases might not be aware that he or she is entering into an implicit Fair Use Agreement when he or she publishes online) and being able to control the distribution of that work. It's especially problematic if the author ever had hopes to monetize it, or sell it to a publisher or someone else.
Of course, most likely exactly none of this is a concern to you. Since the professional publication of fanfiction is a complete non-starter anyway (well, with notable exceptions) public message boards or other self-publishing outlets like Fanfiction.net are the only way for most fanfic authors to distribute their works. Or maybe you simply want to share a bunch of story ideas and you simply do not wish to monetize them. On the other hand, I do recognize that there are members who do have serious hopes to eventually monetize their IPs. If that's the case, then, hopefully you're better informed of how to protect your IPs.