1. Have some sort of indication that something is going on in the back ground.  For example when I change from showing 100 records per page to 1000, it takes quite some time, but I don't always see in the browser anything going on.  Sometimes on the bottom it will say connecting to xx server, but not consistently.
2. The ability to clear all mailboxes selected in mailbox selector.  I will get a request, run it, it generates 100000 hits, the requestor wants to limit to 4 mailboxes instead of all, (over 2200) and I have to redo the whole query because it would take months to delete all the selected mailboxes.
3. Create a button that lets me clear the search query.  If I am doing several searches in a row, I have to log out of retain and log back in because their is no easy way to clear the query (and mailboxes) and start over.  This kind of relates to 2, and perhaps just fixing that would be enough but a button to clear mailbox selection and a button to clear search query would be great.

Those were simple usability.  These are a bit more complex.

1. Most of my search requests are for email between two or more people on a subject.  For example, email between fred@mycompany.com, joe@mycompany.com, and lisa@notmycompany.org.  Currently this is difficult to do in retain  The inability to use the darn @ sign is just part of the problem but its a huge part.  
2. I would like to see more fields brought into mailbox selection from (in our case) exchange.  We currently have department, but things like Company, and other attributes we put in AD would be very helpful in selecting users.