There are only a few keyboard shortcuts in the SpeedGrader, but one that I feel is missing is a keyboard shortcut to submit the comment that you just wrote.
For instance, in google docs, to submit a comment, you type ctrl-Enter or command-Enter.
Currently, in the SpeedGrader, you need to mouse-over Submit and click. That slows me way down! I suggest that there be a keyboard shortcut in order to submit comments.