Slideshow of all images: Not possible currently but on the list for some time. michaell's idea should actually work.
top rated images etc: Not possible currently. But will put it on the list for some time later.
Selected galleries: Well, you mean actually "albums". This is possible with one album at the time. See the documentation. You can even set a start image of that album specifially. That could also be randomly if you pass an image object generated by getRandomImageAlbum().
Besides this the slidesshow uses the jQuery Cycle plugin you can of course make your one slideshow with.
Right, before I thought of the idea of using dynamics though earlier in the thread (I hadn't thought of the slideshow much honestly). However, if you are able to pass the album object to the slideshow directly, you could then just have a direct link to slideshow for a random dynamic album or full album.
Now if only I can figure out an easy way to tag all photos in an album at once.. lol
[i]Slideshow does not show unpublished items by design.[/i]
Not exactly true, if you use the method to call an album with the slideshow directly (not from within an album) it does play unpublished albums, too.
How ever I just discovered a bug with that as calling the slideshow directly on index.php disables the next_album loop for some reason...
EDIT: Will be fixed in tonight's nightly.