It's the app we probably use the most, and Mail is getting some big-league features in Lion. Attendees at WWDC only got a brief glimpse of the new Mail app, but it's loaded with new and compelling features.
The app has been completely redesigned, with a new multi-column view that looks a bit similar to Mail on the iPad. The app can intelligently select recipients as you type, and even suggest subjects as it parses your older mail. There are threaded conversations, and those can be easily saved outside of Mail. Threads even contain attachments.
We'll have more on Mail as we learn more, but with Lion itself selling for US$29.99, the Mail upgrade alone seems like it's worth that.