i'm only 23, but knowledge gaps I've seen in friends/coworkers without degrees are compilers, what needs to happen to go from c to machine code, and to a lesser extent calculus/linear algebra, discrete math, and the stuff covered in an upper division algorithms class.
I think it depends on the person. I'm self taught but I know the rudiments of compiler design and significantly more mathematics than is covered by a standard comp sci course which tend not to get into topology, analysis, or set theory.
