Yes, you could use a Raspberry Pi for a NAS. You would just need to install Samba if you wanted to serve files to Windows machines. If your clients were running Linux or OS X you would probably use NFS and this may be installed by default, but if not the packages are available.
Since the Raspberry Pi has GPIO it might also be fun to see what hardware hacks could be done too. For example, you could have it drive an LCD display or LED bargraph which tells you how much space is left on the drive, or how much I/O is being done. Or perhaps to signal an alarm or flash an LED when the drive is nearly full. It shouldn't be too hard to write scripts or small programs which get this information and drive the GPIO pins.