So first a disclaimer: I'm not a pathologist. I am, however, married to one--and I am a scientist working in areas close to digital pathology.
As far as I know, there aren't that many digital pathology experts in academia--I'd guess you are interviewing at Ohio or Pittsburgh.
Anyway, you can ask about methods of validating that reader performance using whole slide images is equivalent to that using glass slides.
You can ask what issues might arise with whole slide images that wouldn't arise with glass slides (for one thing, what if color isn't reproduced faithfully? What if critical aspects of the image change with the display you are viewing it on?)
You can ask about the FDA regulatory status of whole slide imaging, and recent developments in the area.
You can ask about computer-assisted diagnosis (CAD) in pathology, and how your interviewer sees it affecting pathology workflow in the future. This forum has multiple threads on that subject, including a recent one.
You can ask the same question about telepathology.
You can ask why pathology is so far behind radiology as far as going digital (for one thing, the data storage requirements for pathology are much higher; radiologists don't need to zoom).
I believe there is a pathology department somewhere in Europe that has gone fully digital. You can probably find it via Googling.
Hopefully that gives you a starting point. It's a fun subject. Good luck!