Apple has quietly killed its SyncServices feature in OS X Mavericks, causing problems for iPhone and iPad users who had been avoiding iCloud while synchronizing contacts and calendars to their Macs. iCloud isn't technically required — Apple notes in a support article that you can still use "CardDAV servers, CalDAV servers, and servers that utilize the Exchange ActiveSync protocol" to get the job done — but all available options require sending information beyond the local network. As many commenters in an Apple support discussion thread note, that's a security concern, particularly amidst a year that has seen countless examples of NSA surveillance.
The company is apparently referring affected users to instructions for downgrading to Mountain Lion, but clearly, that doesn't work as a permanent solution as computers and phones get replaced. As one commenter says, "This was hardly publicized. I would have never upgraded."