What makes an online discussion feel serious rather than merely polished?

Good typography is easy. Seriousness comes from incentives.

A serious forum makes it costly to contribute nothing. It has enough continuity that people remember previous arguments, enough moderation that repetition is discouraged, and enough friction that posting is a deliberate act.

The visual layer matters because it signals expectations. But if the underlying contributions are interchangeable, no amount of visual restraint will make the place intellectually durable.