Matt Yglesias complains that federalism doesn't always lead to policy diversity: as he points out, in their remaining areas of jurisdiction, American states often do the same stuff as other American states.
Yglesias's commenters point out that he leaves out a lot of important counter-examples -- the death penalty comes to mind.
But the bigger point is that policy uniformity in matters of provincial/state jurisdiction is the system working as it should. On a lot of issues, the preferences of the median voter will be the same everywhere. On other issues, there are market pressures to do things the way the others do. What Yglesias's point shows is that even when the benefits of uniformity outweigh the benefits of diversity, the lower-level units are likely to deliver it. As a result, one of the main arguments against federalism/for high degrees of centralization is wrong.
The phenomenon of policy convergence demonstrates one of the fallacies of asymmetrical federalism fans. They correctly point out that Quebec is different from the English Canadian provinces in more ways than they are different from each other. But this doesn't mean Quebec needs different powers, just that it is likely to use the same powers differently.