Yesterday, I wanted to sort the 4,000 messages in my Deleted folder by the To field. Now, that field's not in the reading pane by default, so I added it, but it still wasn't there. Using the Arrange By option to sort by the To field wasn't useful if I couldn't view the actual To items. The problem wasn't Outlook but me. Outlook added the To field to the reading pane, but I couldn't see it, because Outlook added it to the far right. The To field wasn't visible, but it was there. Eventually, I found the To field and dragged it to the left where I could see it.
This is just a reminder not to get comfortable with what you see in Outlook -- there's always stuff off screen. If you know it should be there, but it's not, off screen should be the first place you look. The best solution is to arrange your panes to avoid off-screen items, but that can be difficult if not impossible. Deleting fields and panes you don't need helps free up space for the items you actually use.
I did learn a great tip from the experience. Thanks to John Bartow for suggesting that I right-click any column title and choose Field Chooser from the context menu. You can add fields much quicker this way, as opposed to running down through the menu commands, which plant the fields several clicks deep into the hierarchy.