You would be surprised to learn how often this happens: the most complicated situations have the simplest solutions. Before tearing your hair out, before rushing to the forums or IRC asking for help, start with the most simple of verifications.

You just spent two hours creating your virtual host configuration. You've saved the files properly and have fired up your web browser to check the results. But did you remember that one additional step? Nginx, unlike Apache, does not support on-the-fly configuration changes in .htaccess files or similar. So take a moment to make sure you did reload Nginx with service nginx reload, /etc/init.d/nginx reload, or /usr/local/nginx/sbin/nginx -s reload, without forgetting to test your configuration beforehand!