Why should Slice be fully implementable within the language? C++ is another systems language that decided that everything should be able to be done in libraries, and we've all seen the complexity that results.

Well, I'll back off slightly from the assertion "fully implementable" because you have the f = s[i] syntax which wouldn't be fully implementable unless you allow the user to define [], which I agree is crazytown.

However, the make() functions for slice and map are the only functions in the language that allow you to pass in a type and get that type back as a return function. Why can't users have that functionality? Generics seem to be the easiest way to get it.

I think I agree, but I'm not good enough at language design to be sure. :) In the current language, all uses of type literals need to be known at compile time; if I could pass a variable containing a type to "make" or use it to instantiate a map, the language needs generics to deal with that, and all of the complications that come with it. The current idiom seems to be (if I wanted my own "make"):

myslice := mymake((MyType)(nil), 10).([]MyType)

which is much uglier than the built-in way. However, I'm kinda okay with reflection-laden runtime tomfoolery looking a bit messy, mostly because I rarely use it, and when I do I like it to stand out and be obvious what is going on.