D is kind of interesting. Wikipedia says it's existed since 1999, but apparently the 1.0 release was only in 2007.
To me the big question is why I should use D over, say, Java. D has "Java-style single inheritance with interfaces and mixins" according to Wikipedia. If you're going to be Java-style, why not just use Java? I'm sure that D's syntax is probably slightly better than Java's. (That's not a difficult achievement.) But does that slight improvement justify throwing away all the existing libraries and code?
On the other hand, Google Go doesn't have inheritance at all. It has duck typing, enforced at compile-time. Its approach to concurrency is not mutexes and condition variables, but channels. I think these two improvements alone justify the switch to another language.
I am curious how the D and Google Go runtimes compare to the JVM. My experience has been that the JVM runs code relatively fast, but starting the JVM itself takes up a huge amount of resources. In comparison, equivalent C++ or C programs have a lower up-front cost. This is one of the reasons why Java web hosting is still more expensive than PHP web hosting, even today. Java has a high up-front performance cost but then does better than PHP over time.