Hi,
The Synchronization Preview window is brilliant, especially when syncing two machines for the first time.
What would make it really good however, is another function added to the right-mouse-click context menu - "Do this now".
If, for example the sync preview window showed that in a particular folder, 4 files would be copied to S3, you could RMC and select "Do this now". The files would be copied, and the sync preview updated. It would make it much easier to incrementally work your way through a sync preview without having to restart an attended sync each time to get a new preview of what was left to do.