Making text fields autocomplete in views exposed filters
Views change elements after form generation, so you should add own after build function:

function nk_form_alter(&amp;$form, &amp;$form_state, $form_id) {
    if (($form_id == 'views_exposed_form') &amp;&amp;  ($form['#id'] == 'views-exposed-form-my-page')) {
      $form['#after_build'][] = 'nk_views_mypage_exposed_after_build';
    }
}

function nk_views_mypage_exposed_after_build($form, &amp;$form_state) {
  $form['title']['#autocomplete_path'] = 'nk/autocomplete';
  $form['title']['#default_value'] = 'abcd'; // this one also doesn't work
//  print_r($form); // I recommend to use dsm function of devel module, instead of print_r.
  return $form;
}