I'd say: Surprisingly little. It can be a bit daunting and scary due to the website being nothing else than walls of texts, which is an issue. But you can follow some good tutorials on YouTube to install it and once that's done? If you install the Sandboxed Play Services from the Apps app, well. It'll look and feel like a standard Android phone.
It's not like Linux, where everyone will swear that it's super easy and that you never have to interact with a terminal... until you inevitably do and every problem you encounter is met by people telling you to input some weird commands in the terminal without ever explaining why or what it does. (I love Linux, but... come on, you know it's true and that it's a problem.)
Android is design to be used by the most basic of users. And Graphene is still Android at it's core. If you know how to use Android, which is practically anyone... well, once you get it installed, you shouldn't have any issues. :)