Keeping data safe
What about blocking save/publish buttons while upload field processing files?
If user will click on it before processing was finished it will make him upload files again. The worst thing when u have carefully selected bunch of files from long file listing to upload. The same UX issue with GridField editable rows.
Also I guess it will be nice if CMS will store fields content into local storage and restore it after refreshing the page in case user accidentally closed browser window