Simplify Panels "Add content to" popup with fewer options for end users
Checkout https://drupal.org/project/Panopoly - they make a lot of customizations to this to improve the ui.

Some more details from the project page:


  Panopoly is powerful base distribution of Drupal powered by lots of Chaos Tools and Panels magic. The distribution is designed to be both a general foundation for site building and a base framework upon which to build other Drupal distributions