In my experience, anything that is focused is good. The worst kinds of forum discussions are more social/informal as there are better services to handle that type of content.
It’s likely if you have a forum you also have a good following on Twitter, or on Slack so try and make sure the forum content remains valuable by encouraging ‘chat’ off-platform (or in a chat area within the forum, like the Discourse chatbox)
Related to these, I think there are cases where the main participant in a discussion is working through a problem that they expect will take them some time to work through.
Doing so out in the open and asking for input/feedback/help along the way can help keep the context of their journey in one place and make it easier for others to have that context available when they jump in to provide help.
There may be examples of this kind of thing in support forums or in learning based communities.
I think another type of discussion that can be valuable to have in a forum are “inclusive decision making” topics.
For example, when you are planning to make a change in a community or in a project a community has interest in, sharing intentions or proposals with or without alternatives and inviting input of feedback for a period of time, then making a decision which can be recorded with in context.
There may be examples of this kind of thing in “meta” categories about how a community governs itself, community projects, and forums that create space for product feedback, for example.
One kind of discussion that’s ideal for a forum, I think, is pre- or post-meeting discussions. If the community has synchronous meetings, I think that using forums as places for pre-meeting or post-meeting discussion is a good way to get people into the mode/habit of using the forum.
The long-term value piece feels key for many ideal forum discussions. Forum discussions provide an opportunity to summarize best tips/resources that allow for more democratization through comments and upvotes than most traditional resource libraries, as well as more nuance and specificity.