The most important thing to me is stability. Companies like QNap do a great job of increasing the featureset, but from looking at forums, they don't always seem well tested. Synology seem to me to have fewer problems, but are considerably more expensive.
My data is extremely valuable to me, so I'd prefer a slower pace of release if it would lead to fewer problems.
I'd like decent RAID migration speeds
I'd like to be able to upgrade the Apache web server to allow for things such as SVN web server.
Upgradability would also be great - memory and/or even CPU (tho this is less realistic for obvious reasons)
rgds