What is "Advanced Mode" in Views 3?
Advanced mode means 


  Set the Views settings to more developer-oriented values.


You can check the definition of this in views module Drush folder views.drush.inc file.

  $items['views-dev'] = array(
    'callback' =&gt; 'views_development_settings',
    'drupal dependencies' =&gt; array('views'),
    'description' =&gt; 'Set the Views settings to more developer-oriented values.',
    'bootstrap' =&gt; DRUSH_BOOTSTRAP_DRUPAL_FULL,
    'aliases' =&gt; array('vd'),
  );


Once drush vd command is executed following variables are set.

  variable_set('views_ui_show_listing_filters', TRUE);
  variable_set('views_ui_show_master_display', TRUE);
  variable_set('views_ui_show_advanced_column', TRUE);
  variable_set('views_ui_always_live_preview', FALSE);
  variable_set('views_ui_always_live_preview_button', TRUE);
  variable_set('views_ui_show_preview_information', TRUE);
  variable_set('views_ui_show_sql_query', TRUE);
  variable_set('views_ui_show_performance_statistics', TRUE);
  variable_set('views_show_additional_queries', TRUE);
  variable_set('views_devel_output', TRUE);
  variable_set('views_devel_region', 'message');
  variable_set('views_ui_display_embed', TRUE);
  $message = dt("Setup the new views settings.");
  drush_log($message, 'success');


You can do it with out using drush command too by navigating to "admin/structure/views/settings"