First of all, a feature to mark part of a conversation unread has been added to the “labs” (the beta test version of Gmail). This new feature will allow you to read a portion of an e-mail thread, then mark all newer messages as unread. I still wish it was possible to mark specific messages within a thread as unread (you can do so when using a desktop e-mail client with Gmail’s IMAP interface, but you can’t do so through the Web interface of Gmail).
The second new feature, which is available to all Gmail users, is the ability to merge duplicate contacts. If you have multiple contacts set up for the same person, you can click a single button and have Gmail attempt to automatically identify all of those duplicates and merge them where appropriate. Maybe you set up a different contact for each e-mail address or phone number the person has; or maybe you set up an e-mail contact through Gmail and a phone contact through Google Voice. With the new feature, you can easily merge them without having to edit each individual contact.