Try to disable your virus program and your firewall and it should work. It must stay disabled everytime you want to launch the game. I guess that Mojang will make changes to make it safer for Windows 8 users.
Thanks! Disabling the Firewall fixed the issue! I hope that this will be a little easier to handle in future updates!
Edit: After reenabling the Firewall after the game had launched, the game failed to recognize the skin.
Have the same problem. This issue is still apparent in the latest snapshot.