First off citizenship and work visas are totally different things.  Citizenship is a big deal.  You should think once, twice or three times before changing your citizenship.  US citizenship is very valuable and if you lose it, it is not easy to regain.  They have made it harder to lose accidentally than it was say 20 years ago, but you should still be careful.  Unless you change citizenship, you will always be a US Citizen no matter where you live or for how long.

Dual citizenship is not a good idea and is definitely frowned upon by the US government.  It means that both countries "own" you in case of war, problems, etc.  It can create serious conflicts for you.  For example, what if both countries draft you?  What if the two countries are at war with each other, and both draft you?  Who will represent you if you' re picked up while in a third country?  Yes, I realize we aren' t likely to go to war with Canada any time soon, but those are the sorts of issues that come up with dual citizenship.  In many cases when you take the citizenship of a new country you also put your US citizenship at risk as well.  Most dual citizens are the children of parents that come from two different countries, not adults who decide to switch after adulthood for just those reasons.  There is little value for most people to have dual citizenship.

Assuming you do not become a Canadian Citizen, there will be some things that you cannot do while in Canada.  You won' t be able to collect Canadian welfare for example or to vote.  Keep in mind that becoming a Canadian citizen is a major process and isn' t even an option until you had been there for at least 3 years as a permanent resident.  A permanent resident is different than a work visa. Citizenship is not something you should consider unless you plan to make your life there.

Yes, work visas have limits and requirements.  I don' t know what the exact conditions of your visa will be, but you' ll have to follow them to the letter.  If you have a one year visa, you either have to renew or leave the country BEFORE it expires.  If it takes 6 weeks to get renewed, you have to leave time for that.  When you get it, you should get a list of instructions with it.  If not, talk to the Canadian Embassy and be sure you understand it.  If you screw up a work visa, you can be deported and not allowed back in that country for several years (if ever).   You may be covered under NAFTA, which has its own rules as well.  Whatever the rules (I can' t say this enough) FOLLOW THEM.  If they require you to notify them if you move, do it.  If they require you to check in, do it.  Make sure you understand how many times you can leave Canada and come back on your visa.  Make sure you understand how long your can leave.  If you mess that sort of stuff up, you can end up stuck and unable to return to Canada.  Your visa will also have a date that you must enter Canada for the first time by.  Don' t miss that date or you' ll have to start the whole process over.

Taxes are complicated when you are an "expat".  You should talk to a US tax guy, not someone who was in Canada as a student.  As a US Citizen, you will still have to pay taxes in the US, even if you don' t set foot here.  If you should meet a Canadian and decide to get married and then bring your spouse back to the US, the first thing they will ask you for is your last 3 years of US tax returns.  Even if you' re not living in the US, the US cares very much about getting their taxes.  The Canadians will of course want their taxes too.  You don' t pay double, but you may pay more than you would think. In some cases you may pay less than you think.  Your tax guy can show you how it would work in your specific situation.

You should talk to your insurance company too.  It is unlikely that a US company will insure your car if you' re living in Canada and it is unlikely that US insurance will meet Canadian requirements for car insurance.  There' s a big difference between visiting for a few days or weeks and living and working in another country.

Salary.  If you' re being paid in Canadian dollars, then it doesn' t matter what the conversion rate to US dollars is since you won' t be spending those dollars in the US.  Essentially, a dollar is a dollar. If you earn it where you spend it, then that' s the cost of living you should focus on.  If you are earning in US dollars and spending them in Canada, that becomes a lot more complicated to ensure that you' re making enough to cover your costs properly and is also more complicated for tax purposes.  You' re going to have to wait and see what the offer is, then see how that amount translates to cost of living in Toronto.  Don' t convert the Canadian dollars and compare to where you live in the states.  That won' t help you at all.

The Culture Shock series is very good and will give you a much better sense of what it will be like living and working in Canada.  Although Canada is very similar to the US, there are some siginficant differences between living in a completely free country and living in an independant commonwealth that belongs to a foreign head of state (Queen Elizabeth).  There are also serious differences between living in a much more socialist welfare minded environment and living in the US where we expect most people to take care of themselves. 

Finally, do you speak French?  If not, I suggest you start learning.  Although everything in Canada is in English and French, the ability to speak even a tiny bit of French will be very important.