As a NAS, you probably would want ECC and a bunch of SATA ports (I'm using a Helios4 for this purpose). You can't use it as a router / pfSense without a second gigabit ethernet. As a media box, you'd want a SATA port and more display out options (and maybe beefier GPU).
But on the other hand, everyone including me has one. (I got one for flashing some SPI chips with Coreboot BIOS). Perhaps the versatility is the killer feature.