Could you explain what features of the language made it that hard to implement?
I would really like to see your general opinion on Ada from a language designer standpoint. Considering you're the main mind behind D - did you draw any inspiration from Ada while fleshing out the language?
I don't recall just what it was that made it hard, other than being so complicated. I doubt I'd feel the same way looking at it now, as I have a lot of practice implementing complex features from writing a C++ compiler :-)
What I find attractive about Ada is the emphasis on writing programs that are checkably correct. I believe that, with the ever increasing complexity of software, better and much more mechanically checkable encapsulation, etc., is the way of the future, as opposed to what I call "faith based programming" where you overly rely on the programmer not making mistakes.
A large focus of D has been and continues to be on being able to write mechanically checkable code.
Ada suffered for years for being an unimplementable language. It was a long time before compiler technology caught up with it, but Ada found it hard to shake that stigma.
C came along at the right time, did not have those problems, and so took off. On the PC, for example, in the early days C was the only possibility other than assembler.