For one, I think it's mind blowing that they didn't have a good system for dependency management in place from the get-go. Things are pretty good now with go modules but it's like they never even heard of languages like Python and the unholy mess that package management was (and still is to some extent) with Python.
The const/type declaration dance to this day, to declare enumerations, even Algol could do better.
Error handling based on comparing strings, to find out the real cause.
I'm not a fan of Go, at all, but credit where due: they did make this slightly better with error wrapping (https://pkg.go.dev/errors#Is). Of course, this doesn't work unless errors are wrapped, instead of blindly concatenated, forcing you to do string matching more often than not. :)