A very large security vulnerability has been found in all OS X operating systems that pre-date 10.10.3. The vulnerability is being referred to as the Rootpipe Privilege. It gives an user the ability to advance their privileges to root on a machine. Root access on machine, means a user has full access and control. So in retrograde from 10.10.2, if you have a machine with OS X, you are affected by this. It is rumored, but not official, that Apple will not issue security patches in previous versions of their OS.
So if you been holding back on upgrading to Yosemite, now seems to be a good time to do so. Apple released 10.10.3 which seems to have resolved this security vulnerability. If you have a firmware password and use Filevault you may want to check out http://www.grivet-tools.com/blog/2015/yosemite-recovery-update-1-0-causing-problems-in-10-10-3/