Even with the influx of cheap mini-PCs such as Intel's Next Unit of Computing (NUC) series, the humble Network Attached Storage (NAS) unit still has its place. However, if you're new to the NAS game and want one for the house, perhaps to file the role of file server, there are a few caveats to consider before you go ahead with your purchase.
Check the internal file system format
When looking at the specifications for a NAS, you'll often notice that it will have "internal" and "external" support for one or more file systems. These include FAT, FAT32, NTFS as well as the Linux options such as ext2 and 3. The internal file system (or systems) is the one to watch out for.
If you already have drives full of data and were planning to just plug them in, you'll more than likely have to reformat those drives inside the NAS and copy the data back over. So, save yourself some time and hassle and pre-copy the data off the drives, or purchase new, dedicated drives for the NAS.
Supported applications and CPU architecture
Modern NAS units tend to run some flavour of Linux as an operating system, which requires a certain amount of processor grunt. Beyond fiddling with the NAS' web frontend, you should be able to use an SSH client to log into the system directly and access the underlying file system.
You may not have a need to do this if the NAS has a software ecosystem -- for example, Netgear's ReadyNAS series (pictured above) -- but if you can't track down the programs you want, you'll have to install (and even compile) them yourself in the usual way for a Linux-based OS.
Unfortunately, this can be complicated by the processor architecture. Again, with the ReadyNAS, the lower-end models run ARM chips, while the higher-end units come with x86 hardware. Obviously, software compiled for one architecture won't run on another, so be sure to do your research here before taking the plunge.
Finally, not all CPUs are created equal. If you plan to have your NAS transcode video and audio, or even run a Mumble server, make sure the processor is up to the task. Some are barely good enough to reliably run the NAS' various services -- HTTP and file-sharing protocols -- let alone whatever else you want to throw on top.
Can it perform?
Probably the most important aspect of getting a NAS -- how well does it handle file operations? Much like low-end routers, cheap NAS units can have trouble with even moderate traffic, be it over Wi-Fi, Ethernet or USB. This is where it pays to read reviews, particularly ones that cover these kinds of benchmarks. Anandtech and Tech Report are places to start, but perusing Amazon reviews can turn up interesting tidbits too.