I’m probably the last person to discover this, but I haven’t seen it mentioned anywhere before.
I just noticed that you can drag the path from the parent-folders button (at the bottom left of a GNOME file manager window) to the file chooser dialog’s location entry, which is convenient when you are opening or saving from an application and happen to have a folder open in a window already.
However, it doesn’t deal well with spaces, for instance. It escapes them as %20 though the file chooser dialog expects them to be unescaped. This could be very useful with a little work.
It would also be nice if the drop worked on the path bar too, and if I could drag from the window’s title bar.