Also, having helped a number of people now through their early days of the language it's also the one biggest thing by far that reliably confuses every single person. And I mean everyone — from first time programmers all the way up to people who've been in the industry for decades and are learning Go as their tenth language. Being forced to put files in certain places is incredibly non-intuitive because there's nothing else out there that requires it. I must have sent the Go documentation on workspaces [1] to two dozen different people at this point.
But on the other hand, once you've grasped the system and are using it, `GOPATH` is surprisingly not bad. It's always obvious where your dependencies are located and which versions are going to be used to build your project. Even better, it lets you very easily drop into those dependencies and add minor changes or debugging lines if you need to. This can be incredibly useful if you're trying to understand how one of them works or think that that you might have found a bug and trying to verify or patch it. A very powerful feature once you know about it.
The new Go modules seem good, and will be a huge improvement in lowering the barrier to entry for Go, but I'll miss the old `GOPATH` style of work at this point.