Gentlemen, we are now in a state of necessity, and necessity knows no law - Reich Chancellor Bethmann-Hollweg

First and formost, you can see the obvious external difference, a fifth drive slot, leading to the 5xxx number instead of the 4xxx in the older one. Guess how many drives the 2xxx and 1xxx models can fit? You may also note a status LED on the drives, and the keener among you will see that it is also notably bigger than the N4100 sitting right beside it.
On the back, there is now an eSATA port for expandability, so 5 drive bays is not a hard limit. It also has 4 GigE ports, up from the two 10/100s found on the older model. Not a bad upgrade.
To support all of this, you need more horsepower, something that was my biggest complaint about the N4100. That model, along with its older 2 series stablemate was powered by an in Intel XScale ARM CPU, clearly pushing the limits of what it could do, especially since it lacked an FP unit. The newer model has a full Celeron M CPU, so it has several times the raw horsepower, and hardware FP. This should do the trick nicely. Just for reference, the 1 series boxes are Freescale PPC based.
So, other than drives and network speed, what does the N5200 bring to the table? The obvious thing is RAID6, made more practical by the added drive. You can now also do RAID10, but that needs an even number or drives, kind of a waste in this case.
The nicest part is my two biggest bitches about the N4100, RAID expansion and level migration, have both been addressed. If you start out with 4 drives, and need more room, you can plug in a 5th to expand the RAID, no more formatting and restarting. You can also add a drive and go from RAID5 to RAID6 should your boss decide that the level of paranoia you display is not enough.
On the network side, they added link aggregation and failover, both welcome in the corporate environment. I am not sure how practical aggregation is, I have doubts that the box has the raw horsepower to saturate a GigE link, but if it does, you can go higher. Link failover is also on of those very nice features to have, especially in a corporate setting.
One other really nice thing they added was USB target mode, AKA the same thing that lets you use a flash stick as a HD. If you want to, you can stick 5 750G Seagate drives in this, and off you go with a 3TB USB stick that supports RAID5 in case you drop it. I shudder to think about how long it would take to fill this at USB speeds though.
Moving right along to the network protocols, we now have support for Apple (AFP) and Linux (NFS). There is also an admin utility for Macs, and as far as I know, this is the first NAS of it's type to do this, or at least the only one I saw at the show.
Last up, there are a whole host of user rights and software features added on. You can do storage limits by folder instead of by user, and there are now several backup options. It now comes with backup software for the client, and you can take a snapshot of the server itself to back it up.
Basically, it looks like Thecus addressed most of the issues I had with the N4100 in the N5200. On the surface, there doesn't appear to be any glaring weak spots, but as usual, we will wait until we get our hands on one be sure. µ