Technically any motherboard can become a NAS, but there are desirable features.

- low idle power consumption since your NAS will be sitting doing nothing most of the time - pretty much any desktop MB will do

- fast networking, 1gbe means ~100MB/s transfers, nicer to have 10gbe. Limited benefits beyond 10gbe in practice.

- enough PCIe lanes to connect enough drives. HDD of course but nice to have a separate fast SSD array plus SSD caching. You might also want a SAS HBA if you are looking enterprise drives or SSDs (and even for SATA SSD you will get a better performance via a HBA than through the motherboard). Some people also want a graphic card for video transcoding

- ECC memory

- IPMI - once you start using it it becomes hard to give up. Allows you to manage the server remotely even when switched off, and access the BIOS via a web interface. Allows you to keep the server headless (i.e. not have to go plug a screen to understand why the server is taking so long to reboot).

I'd say a good candidate for a NAS motherboard would be something like a supermicro X11SSH-LN4F, you can find used ones pretty cheap on ebay.