When you give a negative rating to any post, this post will be hidden / minimized from you and only from you. But If the post will accumulate more negative votes from different users, it will be hidden "globally". Of course you can always unhide any post manually.
If you don't like the idea of other users hiding some posts from you with their negative votes, you can turn that off using mentioned "hide all posts with negative rating" option. This will unhide all posts that were automatically hidden.
(As you can see the option's name is pretty unfortunate and causes confusion here - we'll rename that in one of the future updates so it will be clear that if affects posts that were hidden due to negative votes from others. That's important, as your negative vote always overwrites the "hide all posts with negative rating" option, so if you have given a minus rating to some post, this post will always stay hidden. We'll probably change that to work both ways, so positive vote will unhide for you a post that was "globally hidden").

A few more notes on the topic:
First and foremost, thank you for all the feedback :).
As our design and dev teams are working hard on some future updates and improvements to the forums and website (surprise, surprise ;), we're looking closely into all parts of it, including the rep & rating systems, to see what could use some fine tuning :). We've seen some interesting ideas in this topic as well as in your emails and it's one of the things that we'll be discussing in our design meetings in the upcoming week.
Now, as with any other system, there's always a possibility that someone will try to abuse it and it's out job to monitor and prevent that. If we'll find users using fake accounts or cheating the system in any other way we'll take appropriate actions.
--
Piotr K.
Design & Dev Team

It doesn't have to be anyone's tomfoolery. Probably someone really wanted to give you the positive rating but clicked the wrong button by mistake. It really does happen, we've received more than a few messages about that.
I cannot tell if we'll solve that problem by allowing users to change their ratings or in some other manner, but this will be fixed :)

I can assure you that we've read the whole topic, no worries :)
PS: We do not post as much as we'd like to, as devs or designers are browsing the forums mostly outside of everyday responsibilities and currently the whole team is working on lots of stuff, but still we read all the feedback / wishes / ideas and take it into account when working on future updates.