I'm with Danny on those. It's been awhile since I looked into that stuff, but I do recall AVN and bone density issues being talked about as common problems for HR candidacy. Moderate to severe displasia seems like it could throw some problems in. It's an interesting perspective, because a lot of us seem to have come at it from the other side, where we expect most ortho's to suggest THR right off the bat and then after doing research you find out that the really experienced HR docs don't actually turn down very many cases for HR.
I'm not sure about the bone on bone deal. I think it all depends on how much bone is available to work with. Probabaly very specific to the individual. I would think a doc could make a pretty good guess just from looking at the x-rays. Like Danny said, I think a lot of folks here have said they were bone on bone for a number of years and still have enough bone and shape to work with.
Good luck