There is so much boilerplate you have to write that productivity drops considerably, both because you have to re-re-re-re-implement things (or use code generation) and because you need to scan lot's of redundant code before making sense of it.
The alternative is often to just use interface{}, which hurts performance and type safety.
This has so far been a theoretical concern for me only. If you want to write reusable data structures you obviously need generics. But none of the code I've written involves reusable data structures, so I can't say I really miss generics.
The productivity level is nowhere near to Python's yet. You should try Go for a couple of weeks and see that still soooo much missing from Go compared to Python.
Improvements to package management is probably the highest item on my wishlist for Go 2.