Remove WYSIWYG editor on all custom post type EXCEPT regular posts

A bit involved, but this should work:

function remove_wysiwyg() {
    global $pagenow;
    if ( 'post.php' == $pagenow ) {
        $type = get_post_type( $_GET['post'] );
        if( 'post' != $type || 'page' != $type )
            add_filter('user_can_richedit', '__return_false');
    } elseif ( 'post-new.php' == $pagenow ) {
        if( isset( $_GET['post_type'] ) && 'page' != $_GET['post_type'] )
            add_filter('user_can_richedit', '__return_false');
    }   
}
add_action( 'admin_init', 'remove_wysiwyg' );