It's a crapshoot what will be an interface and what will be a struct, which is one of the fundamental features of the language.
Returned error values are sometimes errors.New (e.g. io.EOF) and sometimes random fmt.Errorf strings that couldn't possibly be handled (e.g. all of tls.go basically), sometimes actual structs (e.g. PathError), and sometimes panics.
If you can't even get error handling right and consistent, what claim do you have to consistency? At least in java it's pretty much all exceptions.
The math library, as mentioned above, is pretty darn iffy.
The existence of both 'path' and 'filepath' is possibly a mistake.
The 'heap' and 'list' in collections both seem like they should be similar, but 'heap' operates on a thing that fulfills its interface and list operates directly on arbitrary objects without some interface needed.
Due to the lack of 'protected', the std library is littered with exported fields you shouldn't use (e.g. gob.CommonType and template.Template.*parse.Tree and so on).
Sure, the library is consistent in many places because the language is so small there's little else it can do but be consistent, but the fact that it falls flat in error handling and has visible warts makes me feel that it's a very broad std library, but far less consistent than, say, Rust's (where everything returns consistent Optional error types) or Java's or Smalltalks. Sure, it's more consistent than the clusterfuck that is ruby and/or python or javascript or (heaven forbid) c/c++, but by no means is being better than any of those an achievement in this category.