Just a few thoughts.
First I like the suggestions as per posted earlier I think all of you that think improvements can be made to mail should feel free to send Apple there feedback:
http://www.apple.com/feedback/mail.html
Ok now, I think its important to note that Apple doesn't call it a Preview Pane, we came up with that on our own, they call it the message area. And for those that don't like the message area should feel free to turn it off, as this might cause them less headaches for the time being. To do so you see that little divider line, either drag it all the way down or double click it and it will snap to the bottom. To bring it back just reverse.
Also I remember reading somewhere that people tend to be more productive when they batch things. And since I don't speak for everyone, I personally feel that its easier to batch my emails if I move them to a new folder, Mailbox or whatever you want to call it. So that I don't have to go overtop of the emails I've already decided I need to reply to.
My workflow is as follows, forgive me if you see bugs, I start my day and batch my emails. Quickly removing junk and chain letters that everyone feels is a must to send me. I move all emails that will require more attention to my Reply Mailbox. Since I only like to reply to emails at the end of my day so that I stay focused on the task at hand. Also its not possible for me to close my email case something that is important comes along that is like say 'I'm you freaking boss, do this now or your fired' so I don't like to get caught up with unread email badges, and sorting through emails I've already sorted through. Anyway so as the day progresses I jump in an out of my email quite frequently and quickly batch it, until a few hours to the end of my day. Then I go through and email the emails I can get to right away, and store the ones that need further research for a later date in another needs further research Mailbox.
Anyways I hope that helps,
Weston