I have a semi-pro WordPress background, in that I developed using it for several years. Mostly plugins for my own use, and small-scale sites. I found the transition delightful, as content in Grav (written in Markdown) is not only CMS-agnostic, it is also much friendlier for end-users. A large failing of WordPress from v3-4 was the neglect of the importer/exporter, making rebuilds and backups a neverending hassle usually ending poorly.
As @flaviocopes suggest, the documentation is a good place to start. Rather than the typical (outdated) technical run-through of functions and syntactical hierarchies it reads as written for the user, and throughout explains its concepts without requiring deep technical know-how.
As I see it the main strengths of Grav is the ease-of-use from Twig - a popular templating engine, with syntax like Jinja and Nunjucks, which makes layering in dynamic content a breeze - and the modular nature of plugins. At its core is elegant in structure, fast in rendering, and creating sites takes as little effort as you would expect from a modern CMS.
Rathe r than the pre-built functionality of WordPress, where one solution must commonly fit all, Grav does not limit you to finding the right plugin for the job but lets you build things yourself when you decide you need something. Flat-file CMS' are secure in nature in that a live server has no essential need of an admin interface, thus files are only updated through secure channels.