carestad

If your site is depending on posts to have their thumbnail images set, this is the plugin for you. Also makes it possible 
to require tags in every new post.
When trying to save a post without at thumbnail or tags and when activated, the user will get a warning and the post will 
be saved as a draft instead.

Post Requirements

This plugin allows you to require posts to have thumbnail set and/or tags.

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  …

Post Requirements vs Require Post Tags