New users must comment when requesting username

You can use a captcha plugin that requires users to type a short phrase or random letters in order to register on your site.

An example of one of these plugins is the WP-reCAPTCHA, found at:

http://wordpress.org/extend/plugins/wp-recaptcha/

Or you can use a plugin such as CIMY User Extra Fields to add fields to a user profile and registration screen:

http://wordpress.org/extend/plugins/cimy-user-extra-fields/