the difference is the size of the hard drive, the color of the outside (black) and one chip on the mainboard (speculated for future IP VOD service).
Speaking of speculation, I speculate though I don't know yet for sure, that a correct larger hard drive will make a 622 think it's a 722. A search for the specific drive in my 622 on ebay found a guy selling one because he upgraded his 622 with a larger drive, I have queried him for the specifications of the new one.