A couple of tiny corks of requests, bobbing in an ocean of more important ones...all right, I'll just get on with it. :)
To add a folder to 'Favorites', more quickly: in a future version, could it be possible to drag from a contact sheet's tab (the tab labeled with the current folder's name) directly into the 'Favorites' pane? Perhaps there's a faster way to do this (I really must read the PDF cover-to-cover), but so far I've done it by right-clicking an image in a contact sheet, selecting 'Show in Windows Explorer...' and then dragging from the Explorer 'Address' bar into the Favorites pane. Not that it's terribly slow that way, but dragging the tab from the top of the contact sheet into Favorites would be less slow. :)
(Dragging into Favorites from the Browse For Folder window doesn't add the dragged folder to Favorites.)
Not that a double-click is a hardship, but: keyboard shortcut request -- with a photo selected in the contact sheet, pressing Enter would open it into the preview window. Enter doesn't seem to have any other function that I've discovered yet.