Opt-In Content Locker is a plugin that allows to lock important content on your WordPress website and display it for subscribed users only. All you need to do is to wrap protected content with shortcodes [optinlocker]...[/optinlocker]. If people want to view this content they must submit their name/email. After submission all locked content become visible. Very simple workflow.
Demo: http://codecanyon.net/item/optin-content-locker-for-wordpress/2809912