Balsa allows aliases to auto-expand when entering a recipient in the 'To' field of a new mail, as well. By right-clicking on the index entry of a message, addresses can be added directly to the default address book. Among the other things Balsa brings to the table are, if you've enabled it, display of HTML mail. It's not the most popular feature in the world among many, but if you're working a job where you just get a lot of HTML mail or have friends and relatives you can't break of the habit, it's a nice feature to have enabled. For multipart mails with a HTML component, Balsa defaults to the plain text version, you can click on an HTML icon at the bottom of the message to see the HTML version rendered.
Balsa can also render graphical attachments.
Again, it defaults to showing an attachment icon, but clicking on the icon presents the image without resorting to an external viewer. Balsa also has a set of command line options that make it more customizable at launch. The print preview display is very clean and very scalable.