For designers, Grav gives a head start with 35 officially available pre-made sites, called skeletons, which are essentially a collection of theme+content+plugins.

So you can pick and dissect those 35 sites locally, and see how themes are integrated in the Grav workflow.

You don't need to know PHP, as you can work more easily with the Twig standard templating language.

It's not static, that means you'll have the option to add forms, ecommerce, whatever to your site, without having to use 3rd part solutions. Still with static-like performance, as site speed is a key value proposition of Grav.

If you need custom functionality, building your own plugins is easy and you already have ~120 officially released to choose from or to use as example.

Also, moving sites around is as simple as moving the user/ subfolder to another site, no database to mess with.

This makes simple, Git-based deployments a breeze.

A core RESTful API is in the plans, in the meanwhile you can use https://github.com/btopro/page-as-data to expose pages as JSON

Our Form Plugin does the job: https://github.com/getgrav/grav-plugin-form/

Form entries are stored in the filesystem, in YAML / json format, in a user/data folder, and they can be visualized through the Data Manager plugin (https://github.com/getgrav/grav-plugin-data-manager/).

Form submissions can be emailed, and you pretty much do whatever you want.

Not having a database is not a limit :)

Because a static site generator gives you a static site :)

A Grav site is much more flexible and powerful than a static site, while still being able to provide static-site-like speed because of its built-in extensive caching capabilities.

We're a helpful bunch on https://gitter.im/getgrav/grav and http://getgrav.org/forum when you're ready to try it out :)

You might want to check out Grav (http://getgrav.org) - they have some nice pre-built sites to get started very quickly, so far really enjoying it!