Allow folders on grid rows, a la Ecco.
Ecco allows placing folders on grid rows, in addition to having folders as columns. That's very handy.
Example: I have folders for each client. Navigation is simple. Ctrl-1 to collapse the row folders to a single line each. Click or arrow down to a client's folder, Shift-Ctrl-9 or double-click to see the client's items.
With more than a few clients, this is very much faster and easier than having a grid for each client or changing filters.