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.