When importing a lot of photos, you can't click "Import" until all thumbnailing is complete.

This would be better changed so that all photos show in the dialog as they do in the normal thumbnail view, i.e. so that you can scroll through to any point and whatever you scroll to is then moved to the front of the priority queue for thumbnail generation.  You should be able to click "Import" at any time, even before everything is thumbnailed, and then the task of thumbnailing should be moved to the main window.  It is a little tedious having to wait for everything to be thumbnailed to be able to hit "Import".

Also, import restarts if you uncheck "Copy file to the Photos folder" (presumably the same effect applies to checking/unchecking the other options).  That is also frustrating if you realize you want to change an option near the end of an import.  If "Include subdirectories" is checked/unchecked, the thumbnails generated so far can be re-used for the new set of directories to be scanned.