First thing I did after installing Windows 8 and running windows update is to try to run Minecraft. Downloaded fine, but when I open it, I get to a point where the play button is greyed out and the bottom right says Welcome Guest, Please Login, but there's nowhere to login.
Click 'New profile' in the bottom-left, give it a name and click 'Save profile'. From the drop-down box in the bottom-left, select the newly created profile and a login screen should appear.
Known bug: https://bugs.mojang.com/browse/MCL-2494
Known bug: https://bugs.mojang.com/browse/MCL-2494
Stu