I use NAS for my personal backup for important files (work, family photos, music)
I use 4-bay storage enclosures for HD contents since I move both files and disks across NMTs.
If you see yourself moving disks around often (i.e. from NMT/HTPC to the storage , or copy/move files between disks, I'd go for the multibay enclosures directly attached to the NMT or HTPC)
A NAS formatted disk in a certain RAID cannot be moved to an NMT without reformat.
For HD playback, I just prefer direct attached I/O (USB 2 and USB 3) over wired/wireless LAN.