Of course, you could always throw my suggested selection process out the window and make your selection based on performance first, with the other factors as secondary considerations.
The key performance criteria for NASes are read and write throughput. Other specifications that you might focus on for selecting a naked hard drive such as access time, seek time, etc. are generally masked by the overhead of moving data across a network. Factors that do affect throughput include network connection speed, file size and file record size.
You can't do much about record size, which is controlled by your computer's OS. But if you're transferring lots of big files, make sure you get a NAS that has a gigabit Ethernet port. Previous generation NASes with gigabit Ethernet ports sometimes didn't have processing power that could take advantage of a faster network connection. But our tests show that all current-generation products can transfer files faster over a gigabit connection.
Other factors that affect NAS performance are your computer's operating system and RAM size. Today's OSes try to use RAM caching (reading and writing to system memory instead of disk storage) as much as possible to avoid performance slowdowns. The more RAM you have and the smaller the file size(s) that you're dealing with, the more likely that the OS will be able to find what it needs in speedy memory instead of having to go out to (much) slower disk.
No matter which selection method you use, you'll find our NAS Charts to be an invaluable aid in your search. And we've recently added search filters to them to help you more quickly find products with the features you want. And, of course, our in-depth product reviews are also a great source to tap. So go forth, and get the perfect NAS for you!