Some related material (in the order it should be read) for those interested:
On understanding types, data abstraction, and polymorphism: http://lucacardelli.name/Papers/OnUnderstanding.A4.pdf
On Understanding Data Abstraction, Revisited: http://www.cs.utexas.edu/~wcook/Drafts/2009/essay.pdf
LtU discussion: http://lambda-the-ultimate.org/node/3668
LtU discussion of Steele's post: http://lambda-the-ultimate.org/node/3702