I find it annoying that Yahoo! Mail doesn't provide more keyboard shortcuts, especially for tasks performed all the time, like going to the previous/next message, or replying to an email. If you are using Firefox, you can fix this right now:
- Install the Greasemonkey plugin (click on the link for the latest version in the box in the upper right section of the page)
- Restart Firefox
- Load my Yahoo! Mail Keyboard Short Greasemonkey script (click on this link)
- Go to Tools / Install User Script and click OK
Known bug: The shortcut list at the bottom of the page appears twice. If you figure why this is, please let me know!