Windows 8.1 Enterprise with Update, fully patched (as of today; so current through 2014-07-08's patches).
Office Pro Plus 2013 64-bit, fully patched alongside Windows.
HP EliteBook 8560w, with all drivers updated per HP's Support Assistant.
I start Lync, and the splash screen comes up, saying "Starting..." and just sits there with the "progress bar" (5 white dots) scrolling continuously.
I have an identical machine, configured identically, that doesn't have this issue.
How I recover this: Delete C:\Users\USERNAME\AppData\Local\Microsoft\Office\15.0\Lync\sip_username@domain.tld folder, reboot 2 or 3 times, and Lync will start normally.
Short of rebuilding the machine, what other troubleshooting is there to do with the Lync client?