Drag-and-drop support allows you to simply attach a file into Gmail composition window by dragging the file from an open window into it. There's not much to it beyond that, but it's an incredibly handy feature to have.
Also, provided you have permission and access to view other people's Google Calendar, you can now view them when sending out an invitation. Under the subject field in Gmail, there's a new Invitation link to invite other people to an event. You can check your invitee's availability without going into Google Calendar to compare times. When you're finished, a preview of the invitation will appear in your email message. After you send the email, the event's automatically added to your calendar and your friend's calendar.
It's a good day to be a Gmail user.
Insert a calendar invitation [The Official Gmail Blog]