simoninedinburgh
Here are some thoughts from someone who is not a coder but who is fairly capable as a hobbyist via arduous research and trial and error, and who really enjoys using Flarum and wishes for its long term success. I am not a spokesperson for this project in any way, just an end-user of a free product.
The biggest hurdles with Flarum for a non-developer are installation, then extensions.
Note, however, that easier installation (both via simpler documentation and simplified install routines) may dump tons of newbies used to installing things like WordPress (that kind of "just work") into the ecosystem, who will then bump immediately into priority issue number two -- extensions.
Installation
A guide to installing Flarum on Shared Hosting would be for the mass market (of forum creators), and should address both where an auto-install is offered through the Softaculous utility provided by many shared hosts, and where it is not.
The Softaculous auto-install is a godsend. The manual approach of uploading a compressed file and expanding it into a directory is a little harder but it can work. The sticking point that needs better documentation is changing the .htaccess file "rewrite rules" to make various file directory set-ups work.
Most simple users want Flarum to be accessible to users from the root domain name (example.com). But this currently requires a complex set of modifications. More complex websites might want Flarum to be available from a subdirectory or subdomain (forum.example.com or example.com/forum). Explaining all the in documentation more simply and directly would be helpful.
Once Shared Hosting users (the mass market for Flarum) can install reliably, I would move to documenting installation on a Linux-based VPS (a much more sophisticated set of users but still not necessarily Linux experts or programmers).
This will likely require explaining the Composer package which is used in Linux to install and administer PHP-based software (even if not used to install, use of Composer is essential to ongoing maintenance of Flarum), and maybe also the basics of using the terminal to issue Composer commands via SSH.
Flarum can be installed on a VPS via Composer, or by uploading and expanding a compressed file. Sometimes the Control Panel software running the VPS has Softaculous and there is an auto-install option available for Flarum (thankfully Hestia Control Panel does).
All of these methods would need to be explained.
It gets a lot hairier configuration-wise if the VPS Control Panel installs Nginx on top of Apache web server or uses it in place of Apache.
In my experience, in these scenarios getting the config files right for security purposes (the .htaccess and nginx.conf files) is extremely complex unless you understand the programming concept of "regular expressions" and also exactly how Nginx works as a proxy for Apache.
These are not Flaum issues per se, but they cause much of the time spent trying to get things to work properly and securely.
Extensions
The architecture of Flarum is a "simple" core (not at all simple under the hood) with everything else left to users to create extensions to do additional things, which many then generously share with everyone. One problem with extensions for non-coders is that they arise out of the GitHub environment, which is for software developers and uses professional software developer lingo and processes.
Suffice it to say very few users of Shared Hosting who install Flarum are going to become GitHub participants in this lifetime or understand what a "commit" or "pull request" are, let alone make them themselves or write their own or modify Flarum extensions.
I use a bunch of extensions and they are awesome, but they are almost universally undocumented or barely documented as to what they do, how they do it and where they show up on a page (unless you read and understand the raw code).
Only through trial and error can you figure out what their various toggle switches do to your forum, and some extensions only have a single on/off switch.
This wouldn't matter much, except that extensions are required to add what most would consider to be basic forum functionality available in all other web forum software. "Likes", embedding videos from YouTube, counting readership of posts, etc. all require finding and adding an extension then toggling the right switches in the settings.
In fact, many times perfect extensions exist but users don't know it. Posts in this forum frequently alert people to the existence of extensions to do things they request to be added as new features.
I think understanding the Flarum philosophy of "minimal core with extensions" concept at the outset of choosing Flarum would be helpful, with examples and maybe even convenient listings of "extension groups" that a newbie should consider installing from day one to get basic functionality equivalent to other popular forum software.
Better documentation of each extension and all of the toggled options would be immensely useful, and I think writing documentation/how-tos requires a very different skillset from coding extensions. Also, many developers of extensions speak different languages, which makes developing a set of documentation even more challenging.
It seems enough that people write the extensions and give them to others for free (or for a modest fee). They have little incentive to also write great documentation.
Anything you do will be a great help to Flarum, and I'm sure there are plenty of us who will proofread, make suggestions or otherwise get drawn in to helping you if you can do this.