I'm having the exact same problem (Authenticating forever). I even reset my password to make sure I wasn't just using the wrong one (Although it usually tells you when you are, it didn't this time; I even tried using the wrong password, and it did the same thing).
This is clearly an issue with the brand new client, so it may take some time for a solution to be found... I hope it is soon. I want to try out the new features they added. :O
PS: I tried running it in compatibility for windows 7 (I'm on Win8.1), and also tried running as admin. I'm not getting any notifications saying that Firewall is blocking it, but I will try disabling Windows Firewall and then try again and report back. If that doesn't work, it's just about the extent of my ideas.
EDIT: Turning off Windows Firewall didn't fix it. Darn. I really wanted to play for once.