This is a really good question.  I would agree with the first response that the economic changes have made them more different.  Brazil is an emerging economic powerhouse ready to make a splash on the international stage.  Oil alone will make Venezuela a world power in the years to come.  Contrast that to other nations that are struggling to provide basic services to their citizens and it is becoming more difficult to lump all of those nations into one category.

Socially, don't overlook the role governments play in setting the tone for the social reality of the region.  Dictators, communist influences , and corruption will play a strong role in defining the social life of some nations while the citizens of democracies will have the freedom to define those roles for themselves.

I'd argue that it's made them less alike.  Countries like Brazil and Mexico have much less in common with those like Bolivia.  While the bigger countries are moving towards international prominence, smaller ones like Bolivia struggle with very different problems than those faced by the larger countries.