I spent about an hour yesterday debugging a Varnish page caching issue. I combed the site configuration and code for anything that might be setting cache to 0 (effectively disabling caching), I checked and re-checked the /admin/config/development/performance settings, verifying the 'Expiration of cached pages' (page_cache_maximum_age) had a non-zero value and that the 'Cache pages for anonymous users' checkbox was checked.

After scratching my head a while, I realized that the headers I was seeing when using curl --head [url] were specified as the defaults in drupal_page_header(), and were triggered any time there was a message displayed on the page (e.g. via drupal_set_message()):

X-Drupal-Cache: MISS<br>Expires: Sun, 19 Nov 1978 05:00:00 GMT<br>Cache-Control: no-cache, must-revalidate, post-check=0, pre-check=0<br>X-Content-Type-Options: nosniff