I used to use D for larger tasks and Python for quicker ones, like processing some text file and so on. One day I realized that I prefered using D even for those smaller tasks, where quick and dirty solutions would do. One thing that helped was that the standard algorithms (from the std.algorithm module) are really useful and compose very well, once you get to know them, and allow solving those kinds of tasks both quickly and efficiently.

Facebook has made a C-Preprocessor. D is great for such command line tooling. The standard library is not as extensive as Python's, but the situation is much better than in C-land. You can also use it as a scripting language.

D has a nice framework for servers (vibed.org), so you can write webservices (REST,JSON,etc). There is no great full-stack web framework like Rails or Django though.

GUI stuff works in theory, but I don't know any non-toy project. Games (OpenGL,SDL) are possible, but no serious project, yet.

Android/iOS does not really work, yet. Same with embedded, standalone, kernel stuff. ARM in general needs more work. For HPC it really depends on the use case (see dlangscience.github.io). Vector support and GPU integration is lacking. I don't know any image, video, or audio processing stuff in D.

Its like C++, but "better"

D is a general purpose compiled programming language. So, it can be used for almost anywhere e.g. C++ can be used.