Judging by their constant use of the words 'simple' and 'flexible', I'd assume simplicity and flexibility.

Honestly curious, are existing PHP frameworks hopelessly complex and rigid to have this problem?

In my experience Code Igniter does a very good job at being flexible, simple and fast.

Perhaps they should have a page explaining how Fuel is different from Cake, Zend and Code igniter.

The most direct comparisson is CI: CodeIgniter was a great framework for PHP4 but it is still very much based in PHP4. They may have started to move towards PHP5 with CI2, but they didn't rewrite it with PHP5 in mind. As such the whole architecture is still dictated by PHP4. Also CodeIgniter doesn't have classes autoloading, HMVC, modular seperation, build in ORM, migrations and a lot of other things we felt were very much missing.

The comparisson to Cake & Zend is a bit apples and oranges. Both are hugemongous frameworks in comparisson, especially Cake has a pretty big footprint. Zend is even a whole other approach in the sense that it is more like a collection of loosly coupled classes, while Fuel is a more intergrated framework. I know for example that some have already been using Zend classes with Fuel, combining the large offerings of Zend with the development base we offer.