I finally bought a 6 disk license this week.
1) Disks are are formatted separately without striping. This was huge - partially because I has spent a bunch of time recently helping a co-worker mess with a 4 disk raid 5 array at work. Every time the thing balked, we would have to redo the raid and recopy the test data. If my unraid box blows up - I just pull the drives out and read them on a different machine. No other raid 5 product can do this.
2) Mix and match sizes and sata/ide. I already had a bunch of 250 gig ide drives sitting around, but wanted to get some newer 750 satas..
3) Boots off usb. I eliminated Openfiler from my list because of this
4) Spins down disks individually - this is just cool. A huge power savings, and less noise.
5) Expandable - I know you can sorta expand a raid 5 software array, but only if you replace all the disks and let it fully rebuild each time, which is just asking for array failure.
I really looked at a bunch of things - Freenas, openfiler, Drobo, Readynas NV + (waaaaaay to expensive). I was seriously considering the drobo, but after looking at a few failure stories online, I had no desire to be tied into a proprietary hard drive format that could not be read on another device.