1) Anonymity by default, and something like indieauth[0] to validate identities for non-anonymous posting, without having to store user account data on the server.

2) Ephemerality by default. Automatically delete old or non-productive threads, and give me the ability to download data associated with my account or delete it at will.

3) Hackability by default. Offer an API to allow third party clients, and add metadata to the HTML to make writing plugins easier.

[0]https://indieauth.com/

Everything I have ever wanted for a forum was either built into, or a module for phpBB [1]

[1] - https://www.phpbb.com/

Ability to remove any content when I delete my account. Some forums allow that, others don't. HN for example has no 'delete my account' feature.