It takes advantage of PHP 5.3 (anonymous callback functions, namespacing, etc) whereas a lot of the other PHP frameworks feel like PHP 4+.

You can swap in components like Redis easily. Setting it to use mock versions of web-based APIs in the development environment. And the code just feels nice to work with.

Also has a nice Dependency Injection pattern, and the Twig template system is wonderful.

Yes, that's sort of the point. It looks like the MVC frameworks you'll see in Python and Ruby, with allowances for and enhancements where they make sense with PHP as a language.

I would call Symfony2 "PHP for adults".