McGill is right in the middle of a very interesting bilingual city. Montreal is safer than comparably size US cities, but is still a big city. There is a lot to do right there. I have heard that students usually move out of dorms into apartments for their second year and thereafter (you are not required to ever live in a dorm). McGill will be academically challenging. It is a large school and they will treat you like an adult – for example if you need help you are going to have to take upon yourself to go looking for it.
The drinking age in Montreal is 18. I have heard that this partly moves the drinking out of the dorms and into the bars and nightclubs.