I use open directory so that the server only has an administrator account (all users, including myself, just have OD accounts). Important partitions (except Boot partition) are all encrypted -- I just want things secure for financial and personal data that's saved on the server.
I also have it do DNS and DHCP (my router is was flakey with those services), VPN and SSH for remotely accessing data, Calendar and Contacts servers (starting now I'd probably just use iCloud, but I've been using the server for over 3 years).
While not actually server services, I also run a Windows virtual machine under Parallels that is accessed via Microsoft Remote Desktop Connection for Quicken. Plex Media Server and PlexConnect (for Apple TV), iTunes (as a server), DDNS client, and iPhoto to build a database that Plex can serve. These require that the Administrator account be left logged in, so when I start the system I have to log it in for all services to be running.
The mini is 3 1/2 years old, started with Snow Leopard Server, and just upgraded to 8GB RAM and Mountain Lion Server. I've got 9 TB of external drives attached. Works great!