railmedia

Very lightweight plugin which if activated, it locks down the website for public access. Visitors will be prompted with the wp-login.php page where they should log in to access the site. Useful for under construction websites.

WordPress Require Login

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.

WordPress Require Login vs Post Requirements