As in many other things in life, this is an option that you can easily leave to the user. There are users who like a flat display order from newest to oldest, and there are users who like infinite threading. But as UX designers it is often our job to empower users with the options that they can chose from.
One of the best implementations on this thought (I’ve seen) comes from the open source learning management system Moodle. They have four different ways to display replies, and I found that very useful and actually switched between view modes depending on which post I was viewing. It’s extremely powerful and as a user you feel in total control.
Moodle’s four display options are:
A discussion thread may be displayed in four ways. Use the pull down menu at the top of each forum discussion to select a display type.
Display replies flat, with oldest first
Some Moodle site formats display the pull down menu in a slightly different position.
Display replies flat, with newest first
The discussion will be displayed in one line and the chronological order from the newest to the oldest. This is the same as the above, just a different sort order.
Display replies in threaded form
Only the post starting the discussion will be displayed in its full form; replies will be reduced to the headlines (including information about its author and date of release) and organized chronologically; moreover, replies will be shifted towards the right so that only replies to the same post were in the same line.
Display replies in nested form
All posts are displayed in their full forms; replies will be reduced to the headlines (including information about its author and date of release) and organized chronologically; moreover, replies will be shifted towards the right so that only replies to the same post were in the same line.