Josh Hartman

Tired of untagged posts? Use this simple plugin to require users to add at least one post tag before saving a draft, updating a post, or publishing a post. This applies to normal posts and may not be compatible with custom write panels or custom post types.

Require Post Tags

Require users to add at least one post tag before saving a draft, updating a post, or publishing a post.  This applies to normal posts and may not be  …

Denis Yanchevskiy

Allows you to make post, page, custom post elements required: title, content, featured image.
After installing the plugin, you need to go to the Settings -> DCO Post Validator to specify the required elements.

DCO Post Validator

Allows you to make post, page, custom post elements required: title, content, featured image.

Require Post Tags vs DCO Post Validator