Writing effective one sentence product descriptions can be boiled down to:

Describe in simple terms a major product feature that either or both:

a) solves the biggest pain point for your customer

b) makes you the most different from your closest competitor

Examples:

https://heapanalytics.com: Automatically track all website and app user actions - no coding required

https://mailchimp.com/: Easiest to use email marketing platform

https://www.pipedrive.com/: Drag and drop interface CRM - view all deals by type at a glance

https://www.getrevue.co - Effortlessly send a weekly newsletter to engage your audience

http://www.artofemails.com/ - Proven prewritten sales email templates