To add to the very important points
@Incis0r listed:
What are your professional goals and how does the school help you achieve them? What do YOU want out of dental school?
Trying to specialize or going into general? Is academia of interest?
(The following are just examples and are by no means an exhaustive list)
General dentistry
Is the school known for producing general dentists with a lot of experience or is a GPR/AEGD generally needed?
Does the school have high requirements for graduation (produce x amount of crowns, etc.)?
Do students find their own patients or does the school do it for them?
Is there a surplus of patients or is it difficult to find enough to meet requirements?
How much time in clinic relative to time in basic science classes?
Does the school help the underserved?
Outreach programs?
What are the first time pass rates for the boards?
Academia/research interest
Does the school do a lot of scientific research?
What is the funding from the NIDCR like?
Do the professors publish often?
Are they studying things you are interested in?
Does the school have specialty programs?