I am hopeful that she will be a wonderful constructionist, but it's the "unknown element" to her that concerns me.
Bush chose an unknown because he felt it was the only way to avoid a messy confirmation battle with a possible filibuster.
There is a difference between the cases you cite and this case. Bush has known Miers personally for a decade. They have worked closely together in a variety of tasks in all that time. So Bush has an understanding of her outlook on a person-to-person level that didn't exist in the Kennedy and O'Conner situations. Conservatives should take that into account before they denouce the choice.