Is there a way that a member can specify the default sort order? I have a user that would like the newest post in a thread on top.

No. And it doesn’t make sense at all to read a conversation backwards. 

54 minutes ago, opentype said:

No. And it doesn’t make sense at all to read a conversation backwards. 

To me it doesn't, to the user it does.

Does it? Why? Have you asked them? 
Chances are, there is some other problem, e.g. they might not know how to reach the last unread entry and so they open each topic from the start looking for the new entries. That would explain why they want a reversed order. But that would still not be the right solution. Instead they should learn how to reach the first unread post to read on from there – in the regular order. 
 

On 05/12/2017 at 2:23 PM, opentype said:

Does it? Why? Have you asked them? 
Chances are, there is some other problem, e.g. they might not know how to reach the last unread entry and so they open each topic from the start looking for the new entries. That would explain why they want a reversed order. But that would still not be the right solution. Instead they should learn how to reach the first unread post to read on from there – in the regular order. 
 

It does for certain discussions, especially where  people want to see the latest comments first on a thread. Just look at how successful Reddit and it's sorting options are.

I kind of wish IPS took a few lessons from Reddit. Starting with nested posts, sort order and a rating system which would tie into that 'sort by' option.

I don't think anyone can argue with the options Reddit gives you and how things flow nicely and allow users to sort comments by newest > oldest / oldest > newest / hot.

3 hours ago, CSSlife said:

 where  people want to see the latest comments first on a thread.

Still no answer to the “why” questions. 

3 hours ago, CSSlife said:

I kind of wish IPS took a few lessons from Reddit. Starting with nested posts …

Yikes!

On 12/5/2017 at 2:23 PM, opentype said:

Chances are, there is some other problem, e.g. they might not know how to reach the last unread entry and so they open each topic from the start looking for the new entries. That would explain why they want a reversed order. But that would still not be the right solution. Instead they should learn how to reach the first unread post to read on from there – in the regular order. 
 

That was exactly the case with one of my users. He hadn't realised that new posts at the top would still involve him having to scroll down to the last read post and then scroll upwards to read them in a way that made sense, so he was no better off. Once I explained how to start automatically from the first unread post, he was quite happy.

On 5-12-2017 at 1:28 PM, ChrisVanMeer said:

Is there a way that a member can specify the default sort order? I have a user that would like the newest post in a thread on top.

This will require a custom plugin. 

On 12/5/2017 at 3:23 PM, opentype said:

Chances are, there is some other problem, e.g. they might not know how to reach the last unread entry and so they open each topic from the start looking for the new entries.

This was the problem with my user indeed. Once I told him how he could go straight to the first unread post, he was a happy chap.

There are some use cases for a reverse chronological ordering of posts within a thread.   I run an online support group where members use the threads to check-in daily.  They like to see that latest post first.  In the 3x version of IPB, they gave you an option in the admin console to set that sort order but took it away in 4x.  My members have been b****ing about it for years (since the upgrade to 4x).  Even lost a many members to another support site where the forum software they were using still allows that.  It's a trivial matter for sure but even now, I still have members asking about when I'm going to get that fixed and how horrible the new sort order is.  I try to tell them it's no different than most of the other forums out there but that doesn't matter.  7000+ members and they want what they want (or at least what I used to offer).