The most important thing for me here, as far as realistic expectations that I hope you guys fix, are:
Allow the message pane on the left to expand vertically to take advantage of those using taller and/or high resolutions displays. This would alleviate one of my complaints about this view, just doesn't show nearly as much information as the old style did (and it's harder to parse, in my opinion).
Fix the scrolling issue so the whole page doesn't move while you're scrolling through the messages pane.
Display or at least expose as variables the current number of messages in the mailbox and the total allowed (if not already exposed).
Provide some mechanism to rename folders you create.
I will probably modify the message pane view to remove the "preview" text to reduce the height of each row, and also unwrap the date. Also, just a general comment, Invision really likes pop-up menus. There are so many on this page!
Also, the upcoming change to allow me to filter based on recipient/sender is very welcome. THANK YOU! Has the speed of the search in the Messenger been improved? I currently have 28,000 private conversations on my forum, and searching is agonizingly slow. Before anyone asks, I often need to reference old conversations, so deleting them is not an option.
You guys are going to love me when I start seriously playing with 4.x.