I hate, hate HATE the thumbs down thing, but without any ability to group threads into practical forums, and with the tagging system being entirely arbitrary, the only way to make the site manageable at all is to vote down topics of little or no interest. So, that's how a positive rating can go to zero and stay there. Most people don't have any interest in most threads and vote them down to get them out of the way. Every thumbs down by another member takes the number down one until it shows zero.
I HATE doing that because most are perfectly good threads that would be of interest to many. But I wonder how many are passed over by people who might have been interested just because they showed zero?
My other major peeve is that once a thread is voted down, there is no realistic way to have a change of heart later on and go back to find it. Unless you can remember the dates or general context, it is lost in a sea of voted down threads with no way to identify it. Some threads that start out uninteresting pick up interest because of the contributions of subsequent members. If we had some way to see what we had voted down and the response to those threads, many might be revisited. I vote down a lot less stuff now for that reason.
And I still feel really bad voting down a perfectly good thread just to keep the site manageable.