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.

Germain-Italic

This is a simple plugin which is designed to help the posters who sometimes forget to write the post content (main editing zone). Once activated, this plugin will force the users to give post content while adding new post to WordPress site. Whenever User tries to publish a post without giving content, it will give an alert to the user and also highlight the content area so that user can see where to write the post content. I have tested this plugin with WordPress 3.5.1 and it works fine. It should work with older versions of WordPress also.

Force Post Content

A simple plugin to force user to write Post content in Add Post page. Based on Jatinder Pal Singh’s ”Force Post Title” plugin.

Post Requirements vs Force Post Content