I agree. Cannot stand forums where is full of closed-minded pricks.
What they offer in terms of forum categories, the activeness of the forum, and the type of content as well as the theme.
If it's active. I then see how friendly the admin is when speaking to his/her members.
^ ^ ^Friendly people and an active forum. It doesn't have to be a big place or have tons of topics, but I like to know people actually post there.
The other thing is that the forum has to offer a light theme too. I cannot stay for more than 15 minutes on a dark theme. For some reason, it strains my eyes even though I have perfect vision.