Quick post today:
Attaching forwarded messages is fine, but please display the message in the same window as the one I get. I had to explain to my grandma that she has to click the paperclip, then the message, and if the sender was extra-sloppy, click a different thing at the bottom.
Scale down those huge image attachments. Even when the person has a broadband connection, images from digital cameras (and large images, in general) should be scaled down so I can see the image without excessive scrolling.
While you're at it, unlike IE and Firefox, do a bicubic resize. Don't say no one notices the quality difference--I notice. It's being downloaded; surely a good quality resize is faster than my internet connection.
Include a "shut off HTML formatting" button. The internet's grown up, now; we no longer animate for the sake of animating, color for the sake of color, and use strange fonts because they're there.
And to all the ISPs who feel the need to sign your outgoing emails with their name, spam's already a problem; let's not put it in legitimate message, OK?