Nothing special -- I'm at the U for grad school, so I knew a fair amount already. Looked at the interview feedback page on SDN, that's about it.
What impressed you positively?
The amount of thought that had clearly been given to provide prospective students with a positive and complete perspective on the school.
What impressed you negatively?
Secretary seemed disorganized.
What did you wish you had known ahead of time?
The day lasts through 2pm. I'd have been able to prepare better if I'd not anticipated being done by 1pm or so.
Also, ~460 people are interviewed; 250 are offered admission. I had no idea that the odds were that good.