Cross-platform usage
Yes, we still have to think about people using Windows or iBling…
Linus isn’t yet to achieve his goal of world domination, so we still need to be able to share with other operating systems. First off, while Rsync is available for Linux and Mac OS, there are ports to Windows and Android with varying levels of compatibility, but no pure Rsync (unless you count running it with Cygwin). Unison is available for all three desktop OSes and is an excellent choice for synchronising files across an internal network. Syncthing is available for the three desktop systems and Android (there are two Android clients in the Play Store with very similar feature sets) with a beta iOS client available recently.
The OwnCloud server is a PHP web application, although running it on windows isn’t supported, and there are clients for Linux, Mac OS, Windows, Android, iOS and even Blackberry (remember those?). BitTorrentSync has similarly comprehensive coverage, except it covers Windows Phone and not Blackberry. The mobile clients all have a basic interface with options, such as choosing to only sync over Wi-Fi to save on data charges. Syncthing and BitTorrentSync can use your phone’s camera to add a share from the QR code shown on the desktop, Syncthing also allows you to share folders on the mobile device with a QR code or by sharing a link. Adding a server to the OwnCloud client involves typing in a URL, username and password, but this is a one off task so it’s not too much of a hardship.