You can read about FreeNAS's features on their website:
http://www.freenas.org/about/features.html As a file and backup server it is superb, even supporting TimeMachine backups from a Mac. It supports, out-of-the box, far more protocols (e.g. Apple AFP, iSCSI, as well as NFS and Windows file sharing) than Ubuntu or Window Home Server and comes with a very good web-based interface. So if file serving is your requirement it's the best choice. With the use of plugins it can also serve other purposes, e.g. as a Plex server, and it will, of course, do simple tasks like FTP, Web, or database serving.
In the end, as has been said, it's just FreeBSD so you can install just about anything you want on it (and that's just about the most stable operating system around). But it's probably not the best choice for a Minecraft server if that's your thing.
It costs nothing to try it, so why not give it a whirl and see what you think. If you don't like it then one of the Linux distros would probably suit your needs. I'd avoid Windows Home Server; apart from the cost it's no longer a current product.