Microsoft Lync Mac crashes on 10.9 Mavericks
Delete

~/Library/Preferences/ByHost/MicrosoftLyncRegistrationDB.xxxx.plist
~/library/preferences/com.microsoft.lync.plist


and check in

~/Library/Preferences/ByHost/


for additional files with lync or Lync in the name (delete them as well). Then restart Lync.