I used to use http://christianserving.org TruePreview to have mail automatically marked as unread until I want to mark is as read. Try navigating around your email box and you find you accidentally click mails as read all the time and miss important stuff. After True Preview not working I found that the Apple Engineering team removed a key feature in 10.9
We can petition to get it back!
File a bug report at https://bugreport.apple.com