This feature is trumpeted in the Adobe manuals, yet it doesn't actually work. I find that this problem has persisted since at least version 5.0, as people were complaining about it then. I have version 6.0, fully updated, winning on a fully updated Windows 8.1 PC. I have all the same problems noted for previous versions of Lightroom, namely:
In Impromptu Slideshow
1) Spacebar stops the slideshow but does not restart it.
2) The left and right arrow keys do not advance or replay images.
3) Pressing number keys to rank images as they are shown , gives an on-screen prompt to say the image has been ranked, but the images are NOT ranked afterwards, only the first image (which is ranked with the last number typed)
4) Flagging and Picking images likewise flashes a message to say images are flagged, but when I return to grid view only the first image in the slideshow has been changed.
5) The escape key does not exit the slideshow, pressing a mouse button seems to be the only way to exist the slideshow.
Ranking images via the slideshow is sold as an important workflow feature in the training documentation.
It seems to be totally broken in numerous ways.
Adobe any updates on this issue ??