The differences that I knew of where higher sample rates (192 kHz) and supposedly better preamps (perhaps op-amp upgrades) in the HD. I owned the original 896 at one time and thought about upgrading, but decided not to. I never liked the pre's on the 896, and from searching the Net forums, the 896HD wasn't a major increase in quality. In general, I like the 896 and thought it was a nice piece.
I think the MOTU site used to have specs on the 896, but this was a couple years back before the now new 896HD Mark 3.