The ota tuner on the 722 is the same.
Different disk size.
Different decode chip. The chip in the one in the 622 does mpeg2/mpeg4 and was suppose to do VC1. The VC1 decoding didn't work. Because of this change the 722 receivers more Video on Demand (dowloaded material) in HD then the 622. The 622 usually only having the one 1080P movie a month, while pretty much all of the other is in HD on the 722, but SD on the 622.
Note the new 722K is of course a log different than the 722. And the 922 is suppose to be coming soon.