I don't think Win+Y is meant to do anything? I could be wrong. Win+Z will only work when you have a "metro" app running and should bring up the app/menu bar. Win+X should be bringing up the traditional tools menu in the bottom left corner when on the desktop though. But you can right click there too which is also pretty fast.
Also just noticed, Win+X/Z only worked when holding the Win key down first, and then pressing Z. Seems a funny set of hotkeys cause of the ease of hitting Win and bringing up the Metro screen instead.
None of them work for me. Also, when i go to settings in that right hand pop up bar the keyboard is greyed out. Trying to run the Device drivers cdc in my external DVD writer drive. But the drive isnt reocgnised by windows 8. try to download some drivers but can't run .bin format. this is an utter nightmare!