What we are looking for here is something like an Instapaper for email. I often open an email - scan in and if I find something important I make this email "unread" again...
That would be interesting- but I think this is something that should be built into mail apps natively. The way people are using email has evolved past it's current functionality.
Gmail has stars and for the Mail.app, I use flags and then have a smart mailbox for flagged items.