You might think that "security" is a concept that only applies to some information, and then that information is either "secure" or "not secure". Essentially what I'm arguing (and I think you are too) is that "security" is a concept that applies to all information, and it's a spectrum of "how inaccessible is it to people that I don't want to have this information" vs. "how accessible is it to people that I do what to have access to this information". Nothing falls outside of that.
So even the contents of your post, this post that I'm responding to, falls under a sort of security scheme that you're not really thinking about. The key thing with this post is, there probably isn't anyone who you're particularly averse to them having access, and you want it to be accessible to the public in general, so security is very light. Therefore, the level of security that Slashdot offers (basically none) is an appropriate level of security. As I pointed out, when you log into Slashdot, you type in your username, which has a security level comparable to the contents of your post. For both of those things, you have to trust Slashdot only a very small, almost non-existent amount, but it's still trust.
Now you might be thinking, why is this trusting Slashdot to put in public information? Well, that's where it gets a bit foggy and complicated. You don't know what they're doing with that information, and you probably don't know exactly what you're disclosing to Slashdot. By your word choice, you might be giving them information about your background. Use "lift" instead of "elevator", and it hints that you're not American. Mention that you went sledding when you were a kid, and it tells us something about the region where you grew up. There has actually been research into identifying the author of an anonymous writing sample by word choice and sentence structure alone, potentially allowing someone to identify all of your posts across various sites and usernames as "written by the same person".
Really, who knows what information you give away when you post something online, but the point is, that is information that you're trusting Slashdot (and the rest of us) to have.
But then in addition, you also give Slashdot your password. You can say, "Well I don't care about that password. I don't reuse it anywhere and so it doesn't constitute trust." I bet that you don't want me to have your Slashdot password, though, because you don't trust what I'd do with it. That means, when you're logging into the Slashdot website, you're trusting that the site is valid and not compromised, and that Slashdot will keep the password secret. The level of security you're demanding may not be very high, but it's higher than what you're expecting from the contents of your post.
In addition to that, by visiting the site, you're trusting that Slashdot doesn't have malicious code that will compromise your computer. You're also trusting them with information about what browser you're using, and what your IP address is. Now you might have your browser set up to be super-secure, not to run any javascript or Flash, to route through Tor, to block tracking attempts, to obscure data about the system you're working on, etc. In that case, then you're trusting Tor, the developers of your browser, etc. to do those things competently.
No matter what, you're trusting some people, to some degree, with some information. It may all be information that you don't care that much about, but sharing it still implies some base level of trust.