Even when I only really need C, the contract programming that D provides is really helpful as it provides a macro free way of doing rigorous assertions etc. in a way that I can turn off without infecting the actual logic code. If a struct's payload cannot be null, for example, getting the compiler to insert checks into all uses in debug builds can be done easily

That and metaprogramming in D is all compile time so you get RSI-free hands, for free.