Strangely that's both a hard and easy question to answer... Some key "aspects" or "features" that set it apart:

* It attempts to minimize the amount of compiler/toolset knowledge a programmer needs to know to get most things done portably.

* It attempts to minimize the amount of work a programmer does to build variations of the same code.

* It attempts to "bake" the knowledge of tools and systems from experts so that non-experts can use those tools and systems.

* And it attempts to do all that by modeling what a programmer is building instead of how to do that building.

But at the end, all I can say is that it's a personal choice for me.. I've tried many build systems. I've had to poke around and debug many build systems. And nothing yet lets me get back to programming faster than Boost Build. Even when that meant having to write a new b2 toolset for emscripten when my one line compile script wasn't enough. It was faster to write that new toolset in less than one hour including the 10 minutes to write the build files for my small, but now rapidly growing, project.. Than to even consider something else.