What is ABI compatiblity? in a nutshell it let's you use libraries from another programming language. In D, all you need is a D file that describes the C/C++ library as a wrapper so the D compiler knows what you are using.

This means D will build upon everything that already exists for C/C++. You can intermix C/C++ and D in the same project if you so choose to. Which is really a big deal because you don't necessarily have to stop and rewrite years of code you might already have to start using D.

Syntax wise D is a kin to C#/Java, so if you know C# or Java, jumping over to using D is just a matter of learning the language specific Gotchas and core libraries. So the learning curve is quite low if you know C/C++,C#, or Java.

to see what D libraries are out there I suggest looking at http://code.dlang.org/