Well, until recently we've managed to avoid the vowel flattening and raising that has been pretty much a constant throughout the US since independence. We're accused of saying "oot", "aboot" and "hoose", but what we hear from our neighbours to the south is "aot", "abaot" and "haose" -- the vowel being a tail-rounded flat "a" to the Canadian ear. (We use the same vowel in "house", the verb form, which is distinguished in Canadian speach by more than just the zeddifying of the ess.)
That being said, one shouldn't be deceived into thinking that there is a single Canadian English dialect or accent -- we have a number of regional variants. The upper crust of both Ontario and Quebec (the former Upper and Lower Canada) both spoke with an essentially North American vocabulary and a "mid-Atlantic" accent (the sort of halfway meeting between RP and mid-American that, say, Deutche Welle would use for its English-language world news service). The lower classes in the same provinces were, like the American south, far more heavily influenced by Cornish, Kentish, Geordie, Scots and Scottish and Irish gaelic -- but without the black feedback loop. Nova Scotia has several accents, ranging from the heavily Gaelic Cape Breton to the German Lunenberg. Newfoundland "English" is a language of its own -- the accent is a Gaelic-heavy mix, but the grammar is very much Irish Gaelic. British Columbia speaks an odd mix of SoCal and Pacific Northwest with a tiny tinge of RP in the mix. Manitoba and Saskatchewan are essentially indistinguishable from rural Pennsylvania, but for an odd metre that leaked in from the local aboriginal languages. And, well, Alberta is a State at heart -- I don't think an American would have any difficulty picking out the new economic immigrants to that province from the natives in a sentence or two. There is a lot of overlap between political borders, of course, but there are several distinct regional dialects.