Defraggler has a great thing - the queue. In my oppinion it would be even better if there are options to change the order of drives in the queue, to add and remove drives from it.
In addition to that it would be fantastic to have an ability to defrag multiple drives at the same time. This won't help if you defrag multiple partitions on the same drive, so and option to choose between queued defrag and "parallel" one may be needed.