If I have to sum up all my (albeit poor) knowledge on Scheme in one sentence and walk on, then that sentence would be "program is itself data."
It looks quite obvious as soon as you've finished the first few pages of SICP, and you've got totally convinced by the authors' claims so easily. But the fact is,
1) It's really hard to understand really. e.g., just yesterday I babbled to my brother that you can't do this in that language, and you can't do that in this language, ... with full confidence, you know. It's easy to remember this sentence, the jewel in CS, but it's not easy to understand it.
2) Even after I understood it, it is still difficult to apply the understanding into work. And if I can't apply the understanding, the truth is that I do not understand it at all.