I first got and read the spec for Ada in 1982, intending to write a compiler for it. I found it to be overwhelmingly complicated, but I had little experience with compilers at the time. I haven't paid much attention to it since. Many other early compiler devs shared my opinion of it, and indeed it was many years before it was successfully implemented.

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.