For certain types of folders, you may find it useful to have the folder list display the total number of items in the folder, not just the unread items (which it does by default).
For example, I set this on some folders that contain messages to certain distribution lists, where there will always be threads I haven’t read (and knowing how many are unread doesn’t help me since it’s not a goal for me to read everything sent to the DL). I do find it useful to see the total number of items in the folder at a glance because when it creeps up too high I will go to the folder and shift+delete the older items so that I don’t hit my mailbox quota.
To enable this, just right click on the folder in Outlook and choose Properties. Select “Show total number of items” and hit OK.