add a feature combining multiple users with multiple monitors and multiple keyboards/mice
so you could have one computer with two monitors, two keyboards, and one mouse that can switch between the two just dragging across the window. use for MMORPG gaming so can have two users logged in simultaneously.

make command-option tab show all open windows in a list that you can tab thru. i know you can already do expose and then hit tab and arrow keys to navigate, but this would just bring up thumbnails of open windows in a simple list like regular command-tab does for programs. so you can do it with one hand...

Page-down does not function properly in Mail. If you are in the three-pane inbox, and click on a message, then want to scroll the list of messages (top right pane), then when you page-down, it doesn't work. It still thinks you're trying to page-down inside the message text area (bottom right pane).

Jumping to a list item by typing the first few letters of its name does not work in Mail. If you are in the message list view, sorted by "From" for example, and you type the first couple of letters of the sender's name that you want to jump to, it just beeps at you.