This is one that if added I think should be turned off by default since it I would imagine that it would impact performance. It could be either a "preview pane" type of setup or something where when you mouse-over a little magnifying glass icon next to each message line it shows a tooltip containing the first 250 characters or something of the message body. I have a bunch of dummies that send me mail where they always leave the subject lines empty or put in generic things like "hey". So any way of previewing would make finding a particular email much faster than having to open each one until I found what I was looking for.
I originally had this including a request to do actions on multiple emails at a time but I'm a dummy and didn't realize that functionality was already there.
