yes the way they charge less is to do 4 or less drives. Possibly even 2 drives. Apple has been working on ZFS for quite some time. By using ZFS, you can have raid 5 like redundancy with snapshots without the need of all of the raid headaches.
Furthermore, expandability could be done either add on modules, or USB ( or even firewire or esata, though don't bet on these last two). This would give the use the ability for more storage without having to implement a whole system for it.
I have noticed that apple likes to add capabilities and functionality into products without the need of complexity and unnessary buttons & ports.
My guess it it would look a lot like a larger time capsule - with only 2 drives (dual 500GB or dual 1TB), 2 USB ports, 10/100/1000 Ethernet & 802.11n. It may also have an airport basestation built into it... although that is about a 50%-50% possibility. Don't believe that a device like this will be as feature rich as the windows home server. It will probably more like an appliance vs a computer. It will have a set of features that it does well, with little to no upgradability. New features will either be an OS-type update or done in new versions of buying the whole system. I believe these things because it is consistent with how apple has built its business with ipods, airport networking, macs & its software.
Finally, a device like this probably has a fixed price point that it can successfully be sold at to get both maximum profitability and not out price itself in the market. I highly doubt it will exceed $800 - for that much you can almost get another macbook. I think 2 models at $649 & $799.