Canada's relatively small population - if we were a nation of 100 or 300 million, with the exposure .ca gets now, imagine what the demand for good names would be like, and the online marketing opportunities to a population like that. Not that a huge population would necessarily be good, for other reasons. If .us had the same use by government and business in the US that .ca has in Canada, then .us would be a much more valuable extension.
A smaller challenge is always .com, because it is just as good to use in Canada as .ca. But this is a lesser challenge because it's hard to find quality names in .com
I don't see many challenges now, .ca is widely used and accepted in Canada, have a small population but internet penetration is high, and better and easier technology will help ensure more .ca domains get developped - which helps. The same rules apply to .com though, good names will increase in demand, and crappy names will stay crappy.