If I could I would also want to be a DAS for faster file transfers at times either FireWire or thunderbolt (which in theory should be possible because either and can be a host device unlike USB)
So far ive found zero pieces of software that claim to have that capability though but there's some smart people here that know better than me so I'm hoping someone can help
I recently had the same dilemma. I wanted to use my Mac Pro as a storage role and use the Mini as the actual computer that I interact with.
You can certainly setup something like a NAS and you can use the Thunderbolt to Gigabit adaptor to have a dedicated connection between your main computer and the file server. The only thing that bothered me with using the Gigabit network is that you are in theory limited to about 120MB/s using Jumbo frames, but in reality you will be more around 60-100MB/s depending on the overhead and efficiency of you transport protocol (AFP, SMB, CIFS, etc).
Firewire 800 will work, but you will be limited to about 80MB/s. But it's a fairly direct connection.
Thunderbolt is the nice one. The problem is finding a computer that has Thunderbolt capability and can store some drives. You can buy thunderbolt equipped motherboards. But getting drivers outside of OSX that allows for networking in still up in the air. You could put OSX on it as a hackintosh and hope that the OSX Thunderbolt drivers work. Bandwidth will be very good on these, easily surpassing a SATA2 bus.
The other option is to use iSCSI protocol that tunnels SATA commands through a network interface. You would need to run an operating system that provides iSCSI "Target" capability. You will then need an iSCSI "Initiator" client software on your Mac to establish the connection. I think it's about $80 for that piece of software, but there is a trial period to see if it works. This is very good option because it allows you Mac to actually control the drives as if they were directly attached to the computer, but they can managed by a separate computer so you can have a big RAID pool if wanted.
So far OS's that provided iSCSI Target capability is Solaris and Windows Server (standard Win 7 & 8 can use the plug in from Starwind). Something to consider here though is that Solaris offers the very exceptional software RAID ZFS. Windows does not offer any native form of RAID, so you would probably be using whats built-in to your motherboard or a 3rd party dedicated board.
Depending on your storage needs (like less then 10TB), you might be better off just getting a USB external enclosure that can hold 3-5 drives and provides RAID5 support built-in. But I would probably limit myself to 3 or 4TB hard drives at that point. With the new 6TB drives you can get a lot of space in a small enclosure, but I would really want RAID6 for those monsters. I do not think a lot of enclosures offer RAID6 at a friendly price point though. And USB3 can provide some pretty good bandwidth, for mass storage purposes. USB2 would be dreadful and you'd see a real world limit of 20-40MB/s.
Good luck to you sir. There are a lot of choices, but not a lot of options
EDIT: To get back to my opening remarks. What did I go with? Well in order to just get it up and running for the time being I decided to keep all my movies/tv shows on my Mac Pro with the big ZFS storage and have iTunes run on that. And then also have iTunes run on my Mini and serve just music. So at the moment I have two servers running in the house until I can figure out my plan. It's not a big deal since Apple TV shows both servers real easy.