The sheer, monstrous size of Canada and the U.S., by itself, has a significant impact on their cultures, and that’s one similarity between them.
In North America, the very size of the nation-states makes it impossible for the countries to have a single culture. It’s simply not possible for a country of such vast size to be culturally homogenous to the extent countries like the Netherlands, Japan, or Ireland are. In a nation-state the size of Canada or the USA it is inevitable that different regions will have different histories and will develop deep cultural divisions. As Bryan points out, the differences in accent in Canada and the USA are dramatic, as dramatic as the differences between Jamaica and England - a Newfoundlander sounds nothing like a rural Ontarian, who sounds nothing like an urban Vancouverite, and I’m surprised Texans can even understand people from Boston.
So in a sense, a key similarity between Canada and the USA is SIZE. The size of the countries significantly impact both nations’ attitude towards cultural difference. I believe Canada and the USA are historically and presently more inclined to define citizenship and inclusion along civil grounds, rather than cultural grounds. You are an American or a Canadian because you choose to be, and because you live here, and because you owe your allegiance here - not because he speak a particular language, dress a certain way, or worship a certain God. Obviously there is always pressure to conform to things even in North America, but I believe the definition of citizenship is more inclined towards the civil rather than the cultural here, as opposed to places like Japan or Sweden. I’m not saying Japanese or Swedes are racists, and I know Sweden has welcomed immigrants with open arms, but I do believe their national identity is based more on culture than ours is.
One of the most common themes of Canadian complaints is that we lack our own culture. This is, IMO, A) factually incorrect anyway, but more importantly B) misses the whole friggin’ point. The whole IDEA to this country is that we don’t define ourselves by prototypically European touchstones of national identity. There is no universal Canadian dress, dance, or diet because Canada is not defined by those things and was never meant to be defined by those things. Canada is defined by civil membership in a country that was, in a very real sense, created to solve specific social and legal problems. To a large extent I believe, as an outsider, that the United States is much the same. A barbeque-lovin’ Texan is very different from a vegetarian in Sunnyvale, CA, who in turn is nothing like Ted Kennedy, who in turn is quite different from a Hawaiian, but they are not Americans because they listen to the same music or talk the same way or eat the same food; they’re Americans because they have chosen to live together under a common definition of how the law should treat people.
And I think this is based, in part, on the size of the countries. It’s also based on their history, of course, but I think size has a lot to do with it. And to my mind, that’s what makes the two countries very similar.