Avoid those pesky blank page with an error message like "please fill out required fields" then loosing your/users comment info. Validation with javascript.

Title Field validation plugin allows you to very easy to add title and content field validations in your wordpress admin panel.

Using WP Live Comments Validation plugin to validate comments form using jquery validation. This plugin allow you to easy way to validate comment form.

Real Time Validation for Gravity Forms increases conversion rates of your Gravity Form using inline validation messages as user types in field.