Queue up multiple drives to defrag, or even defrag two drives at once. I'm sure two different hard drives could easily pull off a simultaneous defrag, and with modern CPUs, I/O ops barely take up much CPU time anyways.
The shut down after defrag option would be useful if I could do it all at once. Why would someone defrag only one of their drives at night when they go to bed (which is when I assume this feature will be used), anyways?
Also, in addition to queuing defrags, it'd be nice if you could also queue Verifies and Free Space Defrags.