Concepts that surprised you when you read SICP?

SICP really drove home the point that it is possible to look at code and data as the same thing.

I understood this before when thinking about universal Turing machines (the input to a UTM is just a representation of a program) or the von Neumann architecture (where a single storage structure holds both code and data), but SICP made the idea much more clear. Scheme (Lisp) helped here, as the syntax for a program is exactly the same as the syntax for lists in general, namely S-expressions.

Once you have the "equivalence" of code and data, suddenly a lot of things become easy. For example, you can write programs that have different evaluation methods (lazy, nondeterministic, etc). Previously, I might have thought that this would require an extension to the programming language; in reality, I can just add it on to the language myself, thus allowing the core language to be minimal. As another example, you can similarly implement an object-oriented framework; again, this is something I might have naively thought would require modifying the language.

Incidentally, one thing I wish SICP had mentioned more: types. Type checking at compilation time is an amazing thing. The SICP implementation of object-oriented programming did not have this benefit.

I didn't read that book yet, I have only looked at the video courses, but it taught me a lot. Functions as first class citizens was mind blowing for me. Executing a "variable" was something very new to me. After watching those videos the way I now see JavaScript and programming in general has greatly changed.

Oh, I think I've lied, the thing that really struck me was that + was a function.

I think the most surprising thing about SICP is to see how few primitives are actually required to make a Turing complete language--almost anything can be built from almost nothing.

Since we are discussing SICP, I'll put in my standard plug for the video lectures at http://groups.csail.mit.edu/mac/classes/6.001/abelson-sussman-lectures/, which are the best Introduction to Computer Science you could hope to get in 20 hours.