Proper threading.
Read/Unread tracking.
Keyboard UI. (And the mouse UI tends to be bad as well.)
Responsiveness. (Especially: page loads which are not instantaneous.)
Killfiles.
They generally have mediocre search functions unless they are entirely visible to Google.
These are off the top of my head.
http://home.httrack.net/~nocem/
Thunderbird does this by default. Reddit breaks threading when sorting by "new". On HN, you have to search the comments page "minutes ago" or "hours ago" to find new comments in an existing thread.
Instead, we went for XenForo, made by the original vB developers. It's got just enough modern technology to make it a bit more pleasant, and it's super fast (faster than vB 3). Migrating vB to that was easy enough too (took a while, but faster than Discourse which needed three days for the basic import and then still had a million tasks to run in the background)
I'm curious why people don't like forums for discussions.