I would like to suggest that the Save and Open dialog boxes be made able to complete at any point during the population of the directory listing.
For example: I have a directory with close to a quarter million files in it that I often need to save files into. It is also located on a Samba share, so it takes quite a while for the full contents to be displayed. Plasma’s Save dialog seems to have great trouble saving a file here until the contents is fully loaded. GTK’s Save dialog does not, it will happily accept that I want to save a file there, quickly check if it exists, and return control to the calling application if it does not. I’d like to see this functionality added to Plasma.