Every year brings changes, but some years really are turning points: 1492, 1789, 1914, and 1989, for example. Does 2011 belong in the august company of such Really Important Years? Probably not, but it definitely qualifies for membership in the second tier of Quite Important Years.
Three big stories ran right through the year, any one of which would have qualified 2011 for membership status. The Arab Spring is an epochal event, even if democratic revolutions may fail in some countries in the end. The euro crisis threatens the European Union with collapse and confirms the shift of economic power from West to East. And the struggle to prevent disastrous climate change was abandoned for the rest of the decade.
The name, it should be noted, is the Arab Spring, not the Muslim Spring, because a majority of the world's Muslims already live in countries that are democratic: Turkey, Indonesia, Malaysia, Bangladesh, Pakistan, and even Iran (in roughly descending order of how democratic they really are). But the Arab countries seemed remarkably impervious to democracy — until it suddenly became clear that they weren't.