The interface isn't changing except for some things that are being reworked
There isn't really a lot to show right now

Could it be that the interface is mostly eye candy? The underhood sections needed the work (ie: IMAP, performance, memory and internal Backup /Restore) and does not change the look and feel of the program.

How many times has the poor IMAP operations come up?
How many times has the internal backup/restore failed and endusers use winrar/zip instead?
Memory leaks? Seems to have gotten much better with 4.2.x
Better searching? Would be a good thing. Advanced search will find by Received date but not sent date. Small thing unless you migrated from another client. TB will set the received date to the date/time of import.

What improvements would you like in the interface? Give examples.

There is much power to the program and making it available to every user is not easy to do (hard to find the menu choice for 'x' lead to the little help button).
Wizard to build regex statements may help a small amount of users.
Themes to change the icons = eye candy and will add bloat to the memory footprint

Have not knowledge as to what is being changed, just an enduser.

there are interface improvements, but nothing major, there are plans this, but not in 5.0, this version is in development for almost 18 months, there is major code rewrite because new IMAP, new Print Preview, new API interface, more than 50 new macros, new Redirect, new Image download manager, new QR code generator, many improvements in message list pane, main window, addressbook, smime etc. So yes, interface is important and Ritlabs knows this, but current interface is polished and interface changes will come later in 5.x serie because this needs new Delphi 2011.