I 'm going to plagurize the first post a bit, because he was dead on for most of the things he said.
- First and foremost, resilience and stability. If it loses your data, Its a failure.
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.
Ease of use: 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: My minimum standard is RAID 5 with a minimum thoughput twice the speed of a single modern Hard Drive. What this means is I can pound the storage device with two machines at the same time without noticing a significant slowdown. Most better quality hardware RAID 5 controllers can do this. We are talking sustained transfer rates higher than 200Mbps.
Media Server: For a home user having a Media server is important.
Allow open development: Foster community development around your device, if someone wants to put their own software on the unit, why not. You'll just sell more.
Polish and Support: If you design the unit as I have suggested you will have one of the most powerful and electrically efficient units around. Keep supporting and polishing the user interface until it is perfect and the hardware is finally holding it back. Its what's made Apple what it is today.
Hardware Suggestions:
- Use an ATOM or the VIA Nano as the CPU (some crafty end users could turn this unit into a mini server increasing its value).
Integrate a 3ware RAID controller and lighten the CPU load. Your performance level would jdramatically increase.
Use a Intel Pro 1000 nic connected to a PCIe bus with user switchable jumbo frame support (gigabit teaming not needed but would be a nice feature to have)-
4 USB ports would be good (placing resetable fuses on those ports like gigabyte does on their motherboards would be better)
Use standard size fans or even better use FDB (Fluid Dynamic bearing fans). Or sell them as an option.
For any led lighting on the outside have a adjustment for the brightness in the Web interface. Some people like to see a nice bright LED, some people find it annoying.
For the overall size and form factor 4-5 drives is idea. (Imagion using five 1.5TB drives in a RAID 5) 
6TB of storage!!! Some people may want more drives but 90% of those out there will not.
Auto expand replace your drive one at a time allowing the unit to rebuild between each upgrade. When all the drives are replaced the NAS will ask you if you want to expand to RAID to the new size available. I think drobo can do this but am unsure if the newest Thecus devices can.