This may be true for locks and cryptography but a message board is not really a 'security system' nor does it fail completely and catastrophically if someone manages to figure out the details of the ranking algorithm or spam countermeasures.
I can't really think of any good forms of 'security through obscurity'. Is the elimination of buffer overflow vulns and sql injections a form of obscurity? Is SSL a form of obscurity?
SSL is based on obscure prime numbers. Another example is user passwords (obscure text). Sessions and API tokens, too. Credit card numbers, garage door openers, and SIM cards all rely on hidden information. Even door locks are a physical form of hidden keys.
But you're right, there are some forms of security that don't require obscurity. For voting systems though, I would categorize them as "cat and mouse" systems, which unfortunately fall into the obscurity category.