Display Custom Field for a Specific Role, but not for Admin

You can use the WP_User class and the has_cap($role) method. The show_user_profile action passes a WP_User object as a parameter to the called function.

http://codex.wordpress.org/Class_Reference/WP_User#has_cap.28.24cap.29

add_action('show_user_profile', 'my_add_extra_profile_fields');
function my_add_extra_profile_fields($user) {
    if ($user->has_cap('subscriber'))
    {
        //Code here
    }
}