"Dominion" was a descriptive name Canada created for itself to describe a country that was not a colony, not a kingdom, not a republic. The "Dominion of Canada" was something new in the world, a sovereign state that looked ahead to an independent future while refusing to sever ties with the past. Several other countries in the British Commonwealth (okay, it was still called the "Empire" then) picked up the label, so it became a gift from Canada to her sister nations.
Now few people use the term. We are just "Canada." The name of our holiday celebrates the country in a bland, obvious sort of way. It lacks the originality and bold character of "Dominion."
Nonethless, it's a great country, and a beacon of hope to tens of millions of people. Have a great Canada Day weekend.
Update, July 7: My old buddy, Link Byfield, offers an Alberta-First view of why "Dominion Day" is not a better name than Canada Day. I rarely agree with Link's logic, but he's always worth reading. Click here for the full scoop.