ASP.NET MVC 2 makes it much easier to integrate validation into web applications. It promotes a model-based validation approach that enables you to keep your applications very DRY, and helps ensure that validation rules are enforced consistently throughout an application.  The built-in DataAnnotations support within ASP.NET MVC 2 makes supporting common validation scenarios really easy out of the box.  The extensibility support within the ASP.NET MVC 2 validation infrastructure then enables you to support a wide variety of more advanced validation scenarios – and plugin any existing or custom validation framework/engine.

Hope this helps,

Scott