The feature that is most important to me is to be able to easily tag incoming emails with a colour that enables me to see what needs to addressed and when. Postbox allows me to do that with a keystroke.... I have 6 different tags (Reply Required, Awaiting Reply, Will Call Back, Complete Etc.) and I can tag an email in the list with with keys 1 through 6. I also have filters set up that tag (colour) an incoming email from particular recipients.

There are two methods of implementing it, one is to colour the type (Postbox does it this way, it looks a little more polished and it is my preferred method), and the other is to colour the entire email "box", which is how Thunderbird renders it.