Really hard puzzles that you had to submit a solution to if you wanted an interview. This was very rare in those days. The problems were original and clever, not pulled from a book or website. Also, the company was in Cambridge, MA and had very close ties to MIT so they could find people like that.
If you got an interview, and did well you got another challenging programming problem to solve on-site. You were given a computer with the programming environment of your choice and as much time as you wanted, as well as someone to ask questions from.
I think it was also important that the company solved hard graph problems in Lisp, with a lot of macro-driven code. If you were a Lisp programmer, you generally wanted to work there.
If you got an interview, and did well you got another challenging programming problem to solve on-site. You were given a computer with the programming environment of your choice and as much time as you wanted, as well as someone to ask questions from.
I think it was also important that the company solved hard graph problems in Lisp, with a lot of macro-driven code. If you were a Lisp programmer, you generally wanted to work there.