What does the Comment Validation plugin do? It adds client-side form validation to your comment form, helping commenters to fill out the form. It doesn’t get in the way, so if you fill out fields as usual you’ll never notice it. It just shows hints and highlights field when something is missing on submit or something has the wrong format, like email or url (which is still optional).
It should work out-of-the box (upload to your server and activate), but testing so far is limited so any feedback of the plugin running in other themes is welcome!