The thing I miss most is that usenet clients will track exactly which messages you have read, so when you come back to a long-running discussion you can immediately see what is new since you last caught-up with it, something which none of the newer online forums seem able to manage.
The splitting of the message hierarchy and message content into separate views is so obviously superior to hacker news style indenting or flat linear lists that it blows my mind no one recreates this in the modern age.