Look closer.
😀
A lot of what you find below may sound corny or cliche.
I would look at the program, extracurricular's, research, community service and other minutia that aren't necessarily essential to your hand skills and then expound on it by relating it to how it'll make you the best clinician/professional/community leader/researcher possible. Some schools underwent curriculum reform and published a detailed account in the Journal of Dental Education. Look at school newsletters or the letter-to-investors for ideas. Look at their notable faculty and look at what they've written. The schools that asked "Why this school?" were easy for me to differentiate from other schools.
Read this before you write about what is expected of a dentist or what role you think a dentist should play. They define the word "professional" in the pdf and I recommend you read it.
http://www.acd.org/PDF/Ethics_Handbook_for_Dentists_(s).pdf
In hindsight, I think you can pick out any activity (research, community service, etc.) that a dental school has to offer and relate it to how it'll make you into your version of the ideal dentist. What is the personality of the generally-accepted, ideal dentist? Conscientiousness, extroversion, and agreeableness are all somewhat correlated with professional behavior according to one literature. Again relate how the dental school will enhance your professional development. Sometimes, it'll be redundant between dental schools but I think that's okay. I think the important thing here is not necessarily differentiating between dental schools but explaining how any aspect of that dental school will help you reach your full potential.
http://www.jdentaled.org/content/69/11/1222.abstract