What is the most important feature that a forum software must have for you to use it?
I think that a software must be highly customizable.
These days, we compete against Facebook and other social media. Features need to rival the user experience in social media or what the members got used to in social media platforms. Likes, Reactions, Mention/tagging, Notifications, Points, Badges etc can be very useful.
User-friendly. Ideally for anybody from beginners to advanced. That's why I went back to Proboards. One it's free, two, it's user-friendly. It's highly customizable by downloading plug-ins from our Library.