As for his particular insight, if you are familiar with his work, that should be enough.
For those not familiar with his work, this interview might be a good starting point:
http://genius.cat-v.org/ken-thompson/interviews/unix-and-bey...
"The aggressive use of a small number of abstractions is, I think, the direct result of a very small number of people who interact closely during the implementation."
"The aggressive use of a small number of abstractions is, I think, the direct result of a very small number of people who interact closely during the implementation."