I suppose it's the difference between the computational model presented by the language verses the computational model presented by the computer. A combination

I suppose it's the difference between the computational model presented by the language verses the computational model presented by the computer. A combination of how many components require some runtime environment to exist and how many disparate language features there are. Not an intuitive definition of simple for sure, but this is in the context of a system's programming language. In terms of just plain simple, I'd say scheme or just lambda calculus even takes the cake.