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."
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...