I've been using a dedicated retail computer as a file server since I discovered network file sharing some 20 years ago. For over half that time I've done the same for my clients (businesses with less than 10 - usually 1-3 - users).
The primary server need for a small business is file sharing. Everyone sees all the same files (depending on access restrictions) in the same drive letter and folder. There's no concern for versioning when documents are emailed around the office. Most critically for the sysadmin, backup is centralized.
No, you don't get local Exchange, some server-installed apps, or automatic workstation imaging. You do get reliable and versatile data storage and hugely simplified administration. And migrating your data to a new server is a matter of copying one folder, configuring users, and repointing shares.
If you think I’m just an old curmudgeon advocating this for everyone, here’s someone else who agrees. Here’s a good article on establishing a small network file server.
http://windowssecrets.com/top-story/a-cheap-effective-home-server-using-windows-8/
Convinced? Now the pull-back. If you think the Win8 features fit you to a “T”, you may want to wait until next summer. This post raises with good warning the point that we should never rush critical services to a new environment.
http://windowssecrets.com/forums/showthread.php/149213-A-cheap-effective-home-server-%E2%80%94-using-Windows-8?p=870808&viewfull=1#post870808
