Why is it so bad?

Well, it's better than every variation of C++, but it's nowhere near as good as D.

D is worthless. It's neither as good as C++ or C# in their respective problem domains. Simply an awkward middle ground.

Couldn't C funtions be imported as external methods?
As for native compilation, how does NGEN fit into it?

JIT conpiler is shit compared to an actual compiler with real optimization.