Whether it has everything built in that I need, how much dev effort it needs to get from where it is to where I want to be, and can I get someone to make it pretty.
I haven’t deployed just a pure forum in a very long time, they almost always had other stuff attached, whether that was a help desk, a wiki, a blog, a bug tracker + source repository (back in the pre GitHub-is-everywhere days), and sometimes like with the roleplay stuff there isn’t a good option on the market for what I wanted. (Even MediaWiki needed custom development for what I wanted to do)
It would depend for me massively on what I intended to do with the community.
My forums in the past have always used either MyBB or phpBB as the base software, but have in all cases always been heavily customized with my own snippets of code and extra bits and pieces I've had developers edit and add in. Of course I've always utilized themes and custom plugins too, as well as the stuff that's freely available, although frequently you'll find with these open-source free platforms that the plugins aren't always of the best quality and will frequently require some editing to make work optimally.
Like I said, it really depends on what you intend to do with your community. Joining Administrata and being an active member here, as well as using a few of the moderation features just when carrying out Content Order-related stuff has shown me that XenForo is a great platform and the moderation features are probably easier to use than on any software I've used before. However, the moderation features wouldn't usually be a decisive factor for me, I'd be much more focused on what the platform can offer in terms of extra features used to engage members on the site, and the additional plugins that Administrata uses, like the Topsites, Directory and Tasks feature, are a great example of that, in my opinion.
I've always stuck with free, non-self-hosted platforms due to not necessarily having the money to use paid forum software, especially now that I only have a part-time job with bills to pay. Being an avid user of the Zathyus Networks platform where I discovered how I could create my very own forum in the first place if I wanted to continue my forum legacy and not abandon all of the codes and themes I coded for the Zathyus Networks platform, I had to move elsewhere. Jcink was a forum software I heard about off and on, but especially during the impending doom of Zathyus Networks. So, I took a peek around the software and it was much like using InvisionFree again since it's based on the same software. I couldn't find many resource forums on the platform that were solely dedicated to resources for the software, so I decided to create one. That was a major selling point for me moving to the Jcink platform, wanting a place to convert my work and others for administrator's use across the software. It seemed to have the basic features and more I needed to create what could be a successful forum, so I stuck with it. Granted, I use a lot of JavaScript to enhance my board even further, but that's deemed to be something done when you use non-self-hosted software.