Go as a C++ replacement should be fine unless you need manual memory management. D as a C++ replacement should be awesome - D has garbage collection by default but manual memory management as optional. I'm surprised the Mozilla team didn't go with D. I don't have anything against Rust, mind. It just seems to me that in general terms D already provided almost everything the Rust designers had in mind.
1. The language has a long history, which by its nature means there is tons of older C++ code around. So while you can write a new program with a small, safe, consistent set of the language features, you will often find yourself reading and calling older code that deals with corner cases and language features you don't understand. (I mean "you" in the general sense, for all I know serviscope_minor knows every revision of the C++ standard backwards, forwards, sideways, and inside out and all of the compiler quirks and corner cases dating back to Stroustrap's first release.) Go, Rust, and D adoption is hindered by the fact that there isn't half a billion or more lines of code in their respective languages in use around the world, but for all three of those languages you won't get bitten by misunderstanding older versions of the code or compiler quirks. They were designed with the benefit of hindsight using C++ as a starting point.
2. C++ has a preprocessor and header files. In the time it takes to compile a three million line C++ program, you can compile a three million line Go or D program and build a two car garage. I think Rust is in the middle, but closer to Go and D for compiler times on large projects than to C++. The C++ preprocessor and header files are fundamental to the language, you can't get rid of them without breaking most older C++ projects. This is a headache that will never go away. This slow compile time is the fundamental reason most websites are not written in C++ despite the fact that C++ demolishes competitors for performance. I expect to see more websites written in Go, D, and Rust as time goes on because they're getting close to C++ for raw performance but the edit-compile-run-test loop time for any website much bigger than "Hello World" is much closer to that of PHP than that of C++.