I know that adding a collection of books in Calibre works recursively so you can't really give a progress bar as there is no set finishing point; however, it would be useful to display a count of the number of files processed so far, with the user able to estimate via their own knowledge how far they've gone. For example, right now I'm importing roughly 600 books but apart from the length of time that has passed, I don't know how many of them Calibre has done.
Just a small but useful possibility