NAS products are equal parts hardware performance and software features. One complements the other in a balanced product. You need more hardware performance to run more software features at the same time.
The ASRock 1U12LW-C2750 ships as a barebones system, so you have your choice of operating systems. IXSystems sells a preconfigured 4-bay NAS with the same ASRock motherboard running FreeNAS (currently in for testing now). FreeNAS is a great option but one that requires some tweaking and deep insight to configure properly. Other options include NAS4Free, Open Indiana, and Windows Server / Windows Storage Server.
We chose Windows Storage Server 2012 R2 since we have several NAS in the lab with this operating system. Windows Storage Server is familiar as well since it's based on Windows.
Windows Storage Server 2012 R2 opens the door for some interesting options inside the operating system. iSCSI is very easy to build, but Storage Pools is the real highlight. Storage Pools also allow us to build arrays with an SSD cache at the front end for very high-performance systems.