A very small but thoughtful feature in Gmail are the little indicators next to messages that you can use to tell at a glance if an e-mail's directly addressed to you, or if it was sent to other people, or if otherwise was sent to a mailing list or bcc.
See https://productforums.google.com/forum/#!topic/gmail/kfKA76EwtwA for a description of how this works on gmail.
Now I do believe that it's possible that you've used gmail before and never noticed these were there, so I think a better design would be in order (maybe a small arrow with different colors? or an icon with different colors?), but I do think this feature is awesome when dealing with big inboxes.