I spent a few nights last week prototyping the kind of forum I'd like to use for supporting my Lumail email client.
The end result was something that was fast, clean, and reasonably well-coded. The only aspect where it falls down is in the looks-department.
I'm still in two minds about whether I need a forum to support the application, so for the moment it is just deployed on a random spare domain:
It can be described as a cross between hacker-news and reddit. (Though the example disables arbitrary tagging, which means the reddit-inspiration is less obvious.)