1). Rolling its own ORM rather than using Doctrine.
2). YATL (yet another templating language). I wish the PHP world would just standardise on Twig.
3). Localisation seems a bit basic at the moment and it's a shame you have to define messages in a PHP array rather than something more portable like YAML.
4). Would have been great if it used Monolog for logging.
On the plus side, I like the clean design and their approach for events looks cool.