From my experiences shadowing, I would say tell me what it is that you DON'T like about your career. Everyone and their mom will hype up their respective career field and tell the aspiring pre-dent what they think they want to hear (like "this is a great career", or really just "you are making a great choice choosing this field"). The reason why this is, is because before I joined the Navy I had a reservist at my hospital job who used to be active duty, and I would inquire about what it would be like to do this specific job in the military. While what he told me was not incorrect, he only told me the good things...because that is all I asked him about. When I was in, I realized all the crappy aspects of the job.
So now, knowing what I know now, I would inquire about the whole picture...the good with the bad. The average pre-dent, I think, isn't going to ask you about any negative stuff. Hell, they want to be excited about choosing this field, gosh darn it. Why dampen that excitement?
That's what I did with my last GP shadowing experience. I basically told him "look, I appreciate you telling me why you're doing what it is you're doing. But no offense, bro, that's what dental school is for...I'm going to learn all that when I get in. Tell me stuff I CAN'T learn in school...something that only an insider in the field would know...what it is about this job you don't like, what about this career is not perfect for you and you wish you could improve but is out of your hands". Stuff like that. That way I could determine if I could "stomach" the crappy parts of the job and if it would be worth it for me to pursue dentistry.
That's what we discussed in between patients outside of earshot of staff and patients. Also, like others have mentioned, the business aspects. Lord knows we won't learn that in school, so tell me about staff (and issues with staffing), how you started and are running this business, insurance, the field in general, starting and maintaining relationships with specialists, and so on and so forth.
Oh, and one more thing.... if they shadow you in the OR, please help me, tell them the very basics about sterility. What to NOT touch and all that. If your students or whoever contaminate the field, it's us peons who have to then re-drape and such, and then we get looks from the surgeon as to why we're not ready when we should be.