Hi there,
A good "Why [school name]" essay will look to state why the school enables the candidates goals and provides specificity regarding the activities and classes the applicant is interested in. A great essay, however, will take that a step beyond that simple approach. A great essay will provide a compelling argument for not only the above, but also why the candidate is right for the school. So to hijack a monumental quote, "ask not only what the school can do for you, but what you can do for the school". That was cheesy - I know, but I couldn't help myself!
For example, are you an involved alum of your undergraduate program? Showcase that to present how you will be an involved and active alum of the MBA program. Do you have a personal passion that the school doesn't offer a club/activity for? Talk about how you'll bring your unique experiences to campus and enrich the lives of others.
One last point. Specificity is key regardless of which way you go but keep in mind there is general specificity and powerful specificity. Did you randomly click through their website for 10 minutes and drop in a few class titles and professor names? Or did you talk to alumni and visit the program where you had a unique and thoroughly exciting experience?
I hope that helps and if you have any questions, please let me know!
Bhavik