In order to overcome some of the shortfalls we encountered with our previous theme engine, we solved problems by creating a Drupal-specific combination of PHPTemplate and giant renderable "Arrays of Doom". Though the current solution is infinitely flexible for those who understand it - we've inadvertently produced a theme layer that is painfully frustrating to use, and very slow to learn.
Twig provides several huge advantages over PHPtemplate, including: Security, Performance (probably), and Ease of Use.
In this session I'll point out some of the specific problems we faced with PHPTemplate & renderables, and show how these issues are being addressed through the move to Twig. There's still a lot to do, but we're definitely making progress. Come see what template files will look like in Drupal 8, and learn how you can get involved in making a better tomorrow for Drupal.