Many languages have the ability to interface with C via FFI, but based on my experience, D's interop is really seamless. In addition, DStep converts .c/.h to .d/.di so there's very less manual work involved.
May be reserved for future article, but C++ interop is where D just excels. No other language can match that!
And plenty of C++ code actually already works in the mainline branch.
You'd be hard pressed to find other languages that are as developed as D and support C and C++ so natively. C is 100%, and support for C++ is definitely growing every year.
The stuff that is guaranteed to work is already documented on dlang.org. However, there has been more work done than was - I haven't checked in weeks - documented e.g. dmd can apparently catch C++ exceptions (but not in release) so the documentation does not mention this.
May be reserved for future article, but C++ interop is where D just excels. No other language can match that!