I went to UofT for undergrad. Cdn Citizen.
1) Its quite competitive there. You're pretty much in class w/ some of the top students in Canada. But its not impossible to do well. Expect relatively large classes in the first 2 years (BIO150, our 1st yr bio course, had 1500+ people when I took it). In 1st or 2nd year, people get high marks all the time, but they are usually a small percentage of the total populations, b/c the population is so large. Also expect your GPA to suffer a little as you progress. This is mainly b/c some of the less academically inclined students gets weeded out (drop out of life sci, pick less-stringent programs), especially for some of the hardcore life-sci specialist programs (we call them the BIG, Biochem, Immunology, and Mol. Genetics, then there's Pharm/Tox, Pathology - notorious as a pre-med program, ...). As a result, more competition. I was in one of these programs and it was not pleasant. Pretty much everyone of your classmates in 3rd and 4th yr had 3.9+ GPA during their 1st and 2nd yr, many w/ 4.0s. And by then, most profs are reluctant to have high class avg b/c then they'd need to write a letter to the dean or w/e explaining why the marks are so "high". So they keep the class avg at around low 70s, but most of the students are around the avg (again, these are the students w/ very high marks in 1st and 2nd yr, so are quite academically competent). In summary, expect competition and you better work your ass off.
2)
Amazing hospitals around. Literally 50 meters from St. George campus (which is quite small and packed), there are the University Health Network Hospitals (Toronto General - known for cardio, Princess Margaret - Cancer, Toronto Western - Neuro). Then there's the Hospital for Sick Children - one of the best pediatric hospitals in the world. Then there's Mount Sinai Hospital. All these huge research/teaching hospitals are within 100 m of each other. So AMAZING research and clinical opportunities. Again, there will be a lot of ppl competing for these.
3) In terms of volunteering. I found the hospital positions are kind of boring and lame. There are also alot of ppl competing for these positions, so there are interviews, blah blah blah, kinda like applying to med school. But there are many other places you can volunteer, so its really up to you to find what you're interested in.
4) Ontario schools can be very stringent w/ GPA and MCAT. So these "stats" make up a larger chunk of their assessment of you. In some schools, if you fail to meet a certain criteria (ex. GPA, a section of the MCAT, say...Writing), you could have won the Nobel and saved a continent and they will not even look at your app. Can you taste my bitterness?

Out of all the schools, UofT med is quite well-rounded in their approach, so they will look at your entire app.
5) Toronto is a nice city. Downtown area is relatively safe, can't say the same about Scarborough. Very multicultural. Food is amazing.
With that said, I've applied to US Schools as an international student and interviewed at a few top 10 ones. Your citizenship plays a huge role at most schools. Even the private US med schools hold only a few spots for international students (you are considered international if you do not have US citizenship or permanent residence). Forget about most US public schools, they won't even consider you if you're an international student (understandable). The one thing that would've made my US application process much easier would be if I had US citizenship.
In summary, UofT is an amazing school not just for its academics, but also b/c of the hospitals around. So many opportunities. However, if attending UofT means abandoning your US citizenship, then don't do it, a citizenship opens many more doors to US schools. If you can retain your US citizenship, then I would recommend going to UofT if you're ok w/ the competition.
US citizenship opens so many doors to US schools that I think it should be your #1 priority.