Summary:
The variable $metadescription in detail pages does not replace the metadescription content block
Detailed Description:
With this in the page template...
{if !isset($metadescription)}{content block='metadescription' label='Page
description for SEO' oneline='true'}{/if}
...And this in the detail template...
{assign var=metadescription value=$entry->fields.metadescription->value}
...The $metadescription from the detail template is never used. The "page
description" from the parent page always supersedes the detail description in
the detail template. Even if the page description is blank and the detail
description is set, the blank page description will appear in the meta
description tag.