Thanks again for the quick responses and accurate answers.  Please do 
not take anything I have said as a complaint as I do appreciate the 
project and hard work very much.

What's there to report?  When jumping a major version, it is the 
admin's responsibility to scan all config files to be aware of what 
has changed.  We can't possibly describe all the minutiae/changes in 
every config file.
Sure, because this is exactly the kind of stuff that can be moved 
around at any time.  Preferences are not locked into any specific group.
No.  Please see the comment in the preferences file.  Previews in 
dynamic view are a performance killer (EVERY message on the page has 
to be parsed, whether it is read or not).  It is up to the admin of 
the server to determine if they want to make this option available to 
users.

Previews will probably go away in traditional view in the near future 
(most likely for IMP 6).  If you want previews you should be using the 
dynamic view.

Thanks for the quick and accurate info via email.  I modified the 
...imp/prefs.php file and all is well now.

For those having this issue you need to modify the section as below in 
..imp/prefs.php to enable this feature:
===========================================================================
// Previews are disabled by default as it can be performance intensive,
// especially without caching.
$_prefs['preview_enabled'] = array(
     // Disabled and locked by default
     'value' => 0,
     'locked' => false,
     'type' => 'checkbox',
     'desc' => _("Enable message previews?")
);
==========================================================================
Did I miss something?  I read a lot before testing the migration and 
never saw anything about this.

Also, when enabled, the control of this feature has been moved to   
Other->Traditional View as opposed to Message Options->Mail Previews.

Perhaps the preference entry can be always present but greyed out or 
otherwise disabled with some note about what must be done to enable it

Duplicate of Ticket #11040.

With all due respect - # 11040 was closed incorrectly.

The message preview in traditional view DOES NOT work/exist.   
Mouse-over only shows a popup with the subject in it which is 
pointless since you are mousing-over the clearly visible subject to 
get a popup of the same text.

Also the Preferences->Mail Previews that controls this option does not 
exist in IMP 5.0.19 Traditional View.

In 4.3.4 Preferences->Mail Previews does exist and the feature works.   
I have tested both 4.3.4 and 5.0.19 on the same server so the 
environment is exactly the same.

Thank you for re checking this.