Rust: Wants to replace C++ as an expressive systems programming language. Syntax derived from C++. Emphasis on const correctness. Makes use of some functional programming concepts. Extensive metaprogramming. No garbage collection by default. Interface based polymorphism. The type system tracks memory ownership helping to prevent some classes of error at the cost of more cognitive overhead. The standard library is also written in a way that makes it hard to break things. The only one of the three that can easily be used to write libraries that can be called from other programming languages. Medium complexity to learn.
Nim: Slightly higher level than Rust. Reference counting garbage collection which can be turned off, but heaps are by default local to threads. Syntax derived from python. Emphasis on const correctness. Extensive compile time evaluation facilities. Object oriented polymorphism (for now). Sadly allows null by default for now. Extensive metaprogramming. Medium complexity to learn.
Go: Hightest level of the three. Traditional garbage collector. Interface based polymorphism. Simple and easy to pick up.
All three have good stories about multithreading. All three do their best to fix warts in C++ in things like declaration mirrors use syntax. All three start with the chainsaw turned off so you won't cut off your foot unless you're doing something genuinely dangerous like writing an OS or compiler.
For me, I'd use Rust for writing security critical or OS level things like kernels, system libraries, important utilities, and web browsers. Nim for small team applications where you can afford to be a bit clever and Go for large team projects where cleverness has to be discouraged.
I'm not a huge expert in any of these so there's probably important things I'm overlooking.
Yes, I think this is what's driving my gut feel. Go feels quite ... hammer-y. Explicit iterators, very few "niceties", etc.
Have a look at some of these in the manual. Gc stuff is in the standard library somewhere.
This is an example I used recently: http://forum.nim-lang.org/t/567
Don't think it counts as low level but templates and macros are also pretty powerful if you need them. http://hookrace.net/blog/introduction-to-metaprogramming-in-...
I was drawn to it because it is high level, expressive and safe, but you can turn all this off when you need to and keep those bits contained. I'm using it with opencl.
Can't really comment on rust and have limited exposure to go.
D somewhat suffers from trying to do everything at once. There are lots of discussions.
(and, on a side note, Go has a little Wirth in it too: it was concepted as "C meets Oberon", but what it borrowed from Oberon was its simplicity, and it hasn't kept nearly as many Wirthian quirks as Nim)
For that reason, I don't believe that Nim & D offer enough advantages and differences over Rust & Go that it outweighs their lack of popularity and hipness.
Of course others will say the same thing about Rust & Go: C, C++ and Java are much more popular and do currently fill basically the same niches as Rust & Go. You have to draw the line somewhere. It just happens that my line is drawn above Nim & D, but below Rust & Go.
Nim & D look like really awesome languages, though. They're just one killer project away from being popular, the way that Rails made Ruby popular. I would not be particularly upset if the situation reversed, if Nim & D got popular while Rust & Go got unpopular.
(I add this not as a refutation, but merely because I found it amusing.)