Good lord - how long's a piece of string?
First and foremost, resilience and stability. Doesn't matter if it goes like the proverbial off a manual excavation implement if that just means it loses all your data REALLY quickly.
Next, compatibility. There are still companies chucking out kit that requires some piece of dedicated software to connect to it. Wrong. Ideally, it should support access via as many clients as possible, be they NFS, SMB, whatever. It should also not require anything more complex than a web browser to configure.
Security; it should be possible to lock down files on the system by user account, and ideally that should be possible through authentication against
AD,
eDirectory,
NTLM etc.
Expandability - and that doesn't just mean increasing capacity with another disk. It should include the capacity to be enhanced with automated replication/backup capabilities, for instance. I have a
Buffalo LinkStation Pro on one site that runs an unattended backup to a
DriveStation hooked up to it via USB. Simple, yes, lots of devices do it, yes, but it's still worth emphasising.
Performance. Of course, I like fast. Who doesn't?
Beyond that, it gets a bit complex; for a NAS in a corporate environment, for instance, I expect a lot more in the way of performance, redundancy and management tools than I do from one in the home/SOHO environment. In the latter case, I might accord a higher priority to capabilities like media streaming.