I think, although I'm not sure, the biggest difference is in the use of the "o" sound. Perhaps some of the other vowels are different, but definitly that one.
If you have heard the Minnesota accent at all in comparison to, say, someone speaking to you from California, you should notice a really distinct difference in the way they say "Oh" "coke" "know" "soda" I think in other parts of the States, the "o" is followed by a "w" sound. But in Minnesota, for example, it's not. This accent is really similar to Canadians'. It might be where it comes from, but I'm honestly not sure of the origin.
"About" is also always funny to hear a Canadian say cause it sounds more like, "a boot"
There are other differences, too, that I can't think of at the moment, but I think the "oh" sound is the most obvious.
They're also a lot nicer. jiji