For an always running thing, choosing a laptop motherboard is a nice way to guarantee all the power saving features will work properly.

Most desktop motherboards won't allow the PCI express bus to sleep for example, using an extra ~4 watts, $8 per year in eco-friendly countries. Across the ~10 years your NAS will probably sit in a cupboard, that's $80, which isn't an awful lot, but probably would have let you buy a bigger SSD or hard drive which would have had more utility.